The OG. The pioneer. The first Forza title that allows you to modify and drive your favorite vehicle around the environment at your own pace, NOT a race track. And about the only Horizon that feels like it has a storyline, where you gradually move up in reputation by besting rival drivers, similar to the olden Need for Speed.
Being the first title, it has some severe limitations on where you can drive compared to later titles (forget destructible fences and actually going off-road), and the smallest map in the franchise, but it marked the start of what's still carrying the Forza nametag as of now, unlike Motorsport.
Also, the beautiful interpretation of Colorado in this game is still quite unique compared to later titles, and probably due to the small map size, the game feels "alive", since traffic density is much higher. If maps are concerned, I'd choose Colorado and Horizon 4's England to be my most favorite ones. To note, I've never played Horizon 5, and don't intend to do so.
Getting this game to run well-optimized on modern systems at 1920*1080 would be a dream, and another chance to relive a classic childhood favorite. Even better if it's a DRM-free implementation like how GOG.com does it.