In order: 3-4-1-2-2*
FH3 is still the most fun game for me, despite the fact that yes, the map's a bit flat and the roads are a bit too wide. But there was quite a bit of variety, each "biome" had its own identity you couldn't mistake for anything else. The car selection was top notch, probably the best out of the 4 titles, the two expansions proved to be immensely entertaining (I think Blizzard Mountain really nailed the feeling of driving in a blizzard, and the Hot Wheels Island was just silly good fun) Some people will argue that the lack of VWs is a negative, but since I'm not a fan of the brand, it doesn't bother me too much.
FH4, despite its obvious flaws, is still a very good game at the core of it, hence why it's second. I personally think the map is the strongest/best one out of the 4. The seasons, while a bit gimmicky, all look and feel very good. Graphically it's a good step above 3 and 2, too. The car list is good enough despite Toyota committing seppuku, and the blueprint system/route creator adds an element of replayability the other titles didn't have. Negatives include the obvious iffy game mechanics they've applied/modified/re-applied and re-modified since launch and the poorest soundtrack of the lot.
Next is FH1, the one that started it all. The story is easily the best one of the lot, because it's the only one with an actual story.

Colorado always felt a bit of an oddball choice, but the setting works and there's some memorable parts of the map, like the Red Rock Canyons. My biggest gripe has always been the lack of "freedom" so to speak while going around the map, along with events that require you to use a specific car that you'll likely use once (the Bentley championship comes to mind) and then leave to collect dust in your garage. The game economy was a bit iffy in the early stage, requiring you to be judicious with how you spend your money and what you spend it on. Coming straight after FM4 and its near 700 cars, the car list felt a bit light with it's 160-ish at launch, a lot of content had been cut out that could/should have been included from the get go.
FH2 comes after, because I never got to grips with it. I know it's a lot of people's favorite, but I can't actually see why, I've grown bored of it very quickly. The map just feels empty, with miles and miles of fields for you to go through with very little obstacles. The fact there's a few cities dotted around the map would have been cool, but I feel like none of them are particularly interesting or memorable. The progression got annoying fast (do 4 races in X city, road trip to Y city, do 4 races in Y city, roadtrip to Z city, and on and on and on) and the lack of events became quite clear when you realise you need to do each event in each of the car/truck divisions in order to complete the game (that's something like 10 times each). Then you reach the end, and there's no climax, just an invitation to redo everything with Ben. Boring.
And lastly, the 360 version of FH2. Least we say about it, the better. Rushed, botched, glitchy, buggy... It makes Project Cars' launch look like a masterpiece.