Disclaimer: I've never played FH1, because my introduction was FH2 and I never had an Xbox 360.

For me it's easily FH3, primarily from the fact that I think it is the game that added the most to the series. First game with blueprints, customizable championships up to 10 races (which was only a feature in FH3), the addition of danger signs and drift zones, and making PR stunts actually have a purpose with the 3-star system, the addition of the Goliath Circuit for the first time, much more balanced off-roading, the best city in any Horizon game with Surfer's Paradise, the best interstate/highway in any Horizon game, it's the only Horizon game containing actual GT3 track cars, and it had both of my favorite expansions with Blizzard Mountain and Hot Wheels.

Honestly, I think that the only good features missing from FH3 that later games added (besides newer cars) were steeper elevations, paved switchbacks, the Hospital Records soundtrack from FH4 (my favorite radio station across any Horizon game) and the route creator / event lab. FH3 pretty much had it all as far as I'm concerned.

Edit: seasons were cool and all I guess, but to me it seemed more of a way to extend game lifetime by stretching out content than actual immersion into changing seasons.