2016 Alfa Romeo Giulia Quadrifoglio Forza Edition
Wheelspin

2007 Alfa Romeo 8C Competizione Forza Edition
Wheelspin

1934 Alfa Romeo P3
Buy Edinburgh Castle

2015 Alumi Craft Class 10 Race Car Forza Edition
Wheelspin

Ariel Atom V8
My Horizon Life (Speed Zone 11)

2017 Aston Martin DB11 Preorder Car
DLC code from select retailers

2016 Aston Martin Vulcan Forza Edition
Ranked Adventure Grandmaster, Wheelspin

1960 Aston Martin DB4 GT Zagato
Barn Find (Autumn)

2010 Audi TT RS Coupe
Wheelspin

1983 Audi Sport quattro
Barn Find

1964 Austin FX4 Taxi
Wheelspin

2013 Bentley Continental GT Speed Forza Edition
Forzathon Shop, Wheelspin

1931 Bentley 4-1/2 Litre Supercharged
Barn Find from Buying Bamburgh Castle

2013 BMW M6 Coupe Forza Edition
Wheelspin

2005 BMW M3 [E46]
My Horizon Life (Drift Run 8)

1988 BMW M5 Forza Edition
Wheelspin

2013 Cadillac XTS Limousine
Wheelspin

2013 Caterham Superlight R500 Forza Edition
Wheelspin

2017 Chevrolet Camaro ZL1 Preorder Car
DLC code from select retailers

2017 Chevrolet Colorado ZR2
Forzathon Shop 20181011, Wheelspin?

1988 Chevrolet Monte Carlo Super Sport
Story (LaRacer 30 stars)

1979 Chevrolet Camaro Z28
Wheelspin

1969 Chevrolet Nova Super Sport 396 Forza Edition
Wheelspin

2018 Dodge Challenger SRT Demon
Story (The Stunt Driver 30 stars), Wheelspin

2017 Ferrari 812 Superfast
Forzathon Shop 20181004, Wheelspin?

2011 Ford Transit Supersport Van Forza Edition
DLC-only, included in VIP membership (included in Ultimate Edition)

2011 Ford F-150 SVT Raptor
Wheelspin

2010 Ford Crown Victoria Police Interceptor
Wheelspin

1993 Ford SVT Cobra R
Wheelspin

1986 Ford Escort RS Turbo
Barn Find

1985 Ford RS200 Evolution
My Horizon Life (Dirt Racing 4)

1973 Ford XB Falcon GT
Wheelspin

1973 Ford Capri RS3100 Forza Edition
?

1972 Ford Falcon XA GT- **** Forza Edition
Wheelspin

1969 Ford Mustang Boss 302
Story (Drift Club 30 stars)

1964 Ford GT40 MkI
Barn Find (Seasonal? - Spring)

2018 Honda Civic Type R
Forzathon Shop 20181018, Wheelspin

2005 Honda NSX-R
Wheelspin

2004 Honda Civic Type-R
Wheelspin

1991 Hoonigan Rauh-Welt Begriff Porsche 911 Turbo
My Horizon Life (Street Scene)

1972 Hoonigan Chevrolet Napalm Nova
Wheelspin

2006 Hummer H1 Alpha
?, Wheelspin

2015 Jaguar XKR-S GT
My Horizon Life (Road Racing 16)

1993 Jaguar XJ220
Barn Find

1961 Jaguar E-type S1
Barn Find

2018 Jeep Grand Cherokee Trackhawk
My Horizon Life (Cross Country)

2016 Jeep Trailcat
My Horizon Life (Cross Country)

2014 Jeep Grand Cherokee SRT
Wheelspin

2016 Lamborghini Aventador LP750-4 SV
Ranked Online Adventure, Wheelspin

2012 Lamborghini Aventador LP700-4 Forza Edition
Wheelspin

2014 Lamborghini Huracan LP 610-4
My Horizon Life (Road Racing)

2008 Lamborghini Reventon Forza Edition
Car Mastery (Lamborghini Miura)

1992 Lancia Delta HF Integrale EVO
Ranked Online Adventure, Wheelspin

1973 Land Rover Range Rover
Barn Find (Seasonal - Winter)

1972 Land Rover Series III Forza Edition
Wheelspin

2009 Lotus 2-Eleven
?, Wheelspin

2005 Lotus Elise 111S
Wheelspin

1997 Lotus Elise GT1
Barn Find

2004 Maserati MC12 Forza Edition
Car Mastery (Maserati 300S)

2018 McLaren 720S Preorder Car
?

2017 Mercedes-AMG GT R Preorder Car
?

2013 Mercedes-Benz E 63 AMG
Wheelspin

2009 Mercedes-Benz SL 65 AMG Black Series Forza Edition
Car Mastery (Merc 300 SL)

1966 MG MGB GT
Barn Find

2012 MINI John Cooper Works GP
Wheelspin

1965 MINI Cooper S
Barn Find

2017 Nissan GT-R Preorder Car
DLC code from select retailers

2002 Nissan Skyline GT-R V-Spec II [R34]
My Horizon Life (Street Scene 4)

2016 Nissan Titan Warrior Concept Forza Edition
DLC-only, included in VIP membership (included in Ultimate Edition)

1995 Nissan NISMO GT-R LM
?, Wheelspin

1995 Nissan NISMO GT-R LM Forza Edition
Car Mastery (Nissan R390)

2010 Noble M600
My Horizon Life (Speed Trap)

2010 Pagani Zonda R Forza Edition
DLC-only, included in VIP membership (included in Ultimate Edition)

1962 Peel P50
Barn Find (Summer)

2011 Penhall The Cholla
My Horizon Life (Cross Country)

1984 Peugeot 205 T16 Forza Edition
Forzathon Shop (Oct 4-11)

1973 Pontiac Firebird Trans Am SD-455
Wheelspin

2019 Porsche 911 GT3 RS
Forzathon Shop 20181004, Wheelspin

2018 Porsche Cayenne Turbo
Ranked Online Adventure, Wheelspin

2016 Porsche 911 GT3 RS Preorder Car
DLC code from select retailers

2015 Porsche Cayman GTS
My Horizon Life (Road Racing 7)

2017 Ram 2500 Power Wagon
My Horizon Life (Dirt Racing 7)

2010 Renault Clio RS Forza Edition
Wheelspin

1993 Renault Clio Williams
My Horizon Life (Street Scene 9)

1980 Renault 5 Turbo Forza Edition
Wheelspin

1973 Renault Alpine A110 1600s
My Horizon Life (Dirt Racing)

2004 Subaru Impreza WRX STi
Wheelspin

1998 Subaru Impreza 22B STi
Barn Find

1962 Triumph Spitfire
Barn Find

2005 TVR Sagaris Forza Edition
Car Mastery (TVR Sagaris)

1998 TVR Cerbera Speed 12
Barn Find

2015 Ultima Evolution Coupe 1020
My Horizon Life (Drag Strip 4)

2014 Volkswagen Global Rallycross Beetle
Wheelspin

2010 Volkswagen Golf R
Wheelspin

1963 Volkswagen Type 2 De Luxe Forza Edition
Wheelspin

2016 Zenvo ST1
Story (World's Fastest Rentals 30 stars)

Current list according to the wiki and personal experience as OP is missing a few of them:

Alfa Romeo P3 (Purchase Edinburgh Castle, automatically added to garage)

Barn Finds:
Aston Martin DB4 GT Zagato (Autumn Exclusive)
Audi Sport quattro
Bentley 4-1/2 Litre Supercharged (Barn Find Rumour: Purchase Bamburgh Castle)
Ford Escort RS Turbo
Ford GT40 Mk I (Spring Exclusive)
Jaguar E-type S1
Jaguar XJ220
Land Rover Range Rover '73 (Winter Exclusive)
Lotus Elise GT1
MG MGB GT
Mini Cooper S
Peel P50 (Summer Exclusive)
Subaru Impreza 22B STi
Triumph Spitfire
TVR Cerbera Speed 12

Car Pass:
BMW i8 Roadster: (Early Gameplay)
Honda Civic Coupe GRC (Early Gameplay)
Koenigsegg Agera RS
Mazda RX-7 Spirit R Type-A
Nissan Sentra Nismo
Porsche #186 Prodrive Rallye 959
Porsche 356 A 1600 Super
Triumph TR3B

Events:
Jeep Trailcat - Cross Country Series level 4
Jeep Grand Cherokee Trackhawk - Cross Country Series level 8
Penhall The Cholla - Cross Country Series level 14
Ford RS200 - Dirt Racing Series level 4
Ram 2500 Power Wagon - Dirt Racing Series level 8
Renault Alpine A110 1600s - Dirt Racing Series level 16
Ford Mustang Boss 302 - Drift Club level 10
Chevrolet Monte Carlo Super Sport - LaRacer @ Horizon level 10
Lamborghini Huracan LP 610-4 - Road Racing Series level 4
Porsche Cayman GTS - Road Racing Series level 8
Jaguar XKR-S GT - Road Racing Series level 16
Noble M600 - Speed Trap Hero level 11
Ariel Atom 500 V8 - Speed Zone Hero level 11
Nissan Skyline GT-R V-Spec II - Street Scene level 4
Renault Clio Williams - Street Scene level 8
Hoonigan Rauh-Welt Begriff Porsche 911 Turbo - Street Scene level 17
BMW M3 - The Drift Run level 8
Dodge Challenger SRT Demon - The Stunt Driver level 10
Zenvo ST1 - World's Fastest level 10
Ultima Evolution Coupe 1020 - Drag Strip level 4

Formula Drift Car Pack:
Formula Drift #98 BMW 325i
Formula Drift #43 Dodge Viper SRT10
Formula Drift #13 Ford Mustang
Formula Drift #530 HSV Maloo Gen-F
Formula Drift #118 Nissan 240SX
Formula Drift #232 Nissan 240SX
Formula Drift #777 Nissan 240SX

Best of Bond Car Pack:
James Bond Edition AMC Hornet X Hatchback
James Bond Edition Aston Martin DB5
James Bond Edition Aston Martin DB10
James Bond Edition Aston Martin DBS
James Bond Edition Aston Martin DBS
James Bond Edition Aston Martin V8
James Bond Edition BMW Z8
James Bond Edition Citroen 2CV6
James Bond Edition Jaguar C-X75
James Bond Edition Lotus Esprit S1

Forzathon Shop Exclusives:
Chevrolet Colorado ZR2 2017 (Winter)
Ferrari 812 Superfast (Autumn)
Porsche 911 GT3 RS 2019 (Summer)
Honda Civic Type R 2018 (Will appear in Spring Forzathon Shop October 19, 2018 to October 25, 2018)

Skill Trees:
Lamborghini Reventon Forza Edition: Found in Lamborghini Miura P400 skill tree
Maserati MC12 Forza Edition: Found in Maserati 300S skill tree
Mercedes-Benz SL 65 AMG Black Series Forza Edition: Found in Mercedes-Benz 300 SL Coupe skill tree
Nissan Nismo GT-R LM Forza Edition: Found in Nissan R390 skill tree
TVR Sagaris Forza Edition: Found in TVR Sagaris skill tree

VIP Membership:
Ford Transit SuperSportVan Forza Edition
Nissan Titan Warrior Concept Forza Edition
Pagani Zonda R Forza Edition

Wheelspin Reward:
2007 Alfa Romeo 8C Competizione Forza Edition
2017 Alfa Romeo Giulia Quadrifoglio Forza Edition
2015 Alumi Craft Class 10 Race Car Forza Edition
2016 Aston Martin Vulcan Forza Edition
1964 Austin FX4 Taxi
2010 Audi TT RS Coupe
2013 Bentley Continental GT Speed Forza Edition
2013 BMW M6 Coupe Forza Edition
2013 Cadillac XTS Limousine
1979 Chevrolet Camaro Z28
1969 Chevrolet Nova Super Sport 396 Forza Edition
2010 Ford Crown Victoria Police Interceptor
2011 Ford F-150 SVT Raptor
1993 Ford SVT Cobra R
1973 Ford XB Falcon GT
2004 Honda Civic Type-R
2005 Honda NSX-R
1972 Hoonigan Chevrolet "Napalm Nova"
2006 Hummer H1 Alpha
2014 Jeep Grand Cherokee SRT
2016 Lamborghini Aventador LP750-4 SV
1992 Lancia Delta HF Integrale Evo
1972 Land Rover Series III Forza Edition
2009 Lotus 2-Eleven
2005 Lotus Elise 111S
2013 Mercedes-Benz E 63 AMG
2012 Mini John Cooper Works GP
1995 Nissan Nismo GT-R LM
1973 Pontiac Firebird Trans Am SD-455
2018 Porsche Cayenne Turbo
2004 Subaru Impreza WRX STi
2014 Volkswagen Global RallyCross Beetle
2010 Volkswagen Golf R
1963 Volkswagen Type 2 De Luxe Forza Edition
1988 BMW M5 Forza Edition
2013 Caterham Superlight R500 Forza Edition
1972 Ford Falcon XA GT- **** Forza Edition (Censored by this site for some reason)
2012 Lamborghini Aventador LP700-4 Forza Edition
1980 Renault 5 Turbo Forza Edition
2010 Renault Clio R.S. Forza Edition
Peugeot 205 T16 Forza Edition

Not sure where found or simply not added yet:
Audi Sport quattro Forza Edition
Austin FX4 Taxi Forza Edition
Ford Capri RS3100 Forza Edition