1. Need For Speed III: Hot Pursuit (1998): Really fun, lots of replayability value. Tracks aren't nearly as wacky as II. I guess it also helps that I grew up primarily playing this one.
2. Need For Speed: Hot Pursuit (2010): Although it's essentially a Burnout game, it still captures a lot of the feeling of the original era of Need For Speed - Exotic cars in exotic locations.
3. Need For Speed: Carbon: Many call this inferior to Most Wanted (2005) but there are quite a few things about it that I like over that game. Cars are split up into three defined tiers and classes which I think makes things a little more organized in car selection. Street racing tends to be a night thing (I'm a little confused as to why Most Wanted took place during the daytime.) No drag racing (that was mildly annoying in Underground 1 & 2, but insufferable in Most Wanted). I also thought the canyon races were an interesting challenge as well. I've beaten this game five times, as opposed to Most Wanted which I haven't beaten once. It just feels like there's less tedium in this game.
4. Need For Speed: Most Wanted (2012): There were a few wacky things they did with this game, with the "upgrades" and the fact that it's very stunt-oriented (billboards, loads of jumps, lots of areas off the main road to explore) but I feel it worked out well. I got this game last month and I found it to be really fun. The entire place felt like a playground, something that none of the previous installments never really felt like. If I would've been like 9 or 10 when I got this game, I'm willing to bet this would've been my favorite in the series.
5. Need For Speed: Hot Pursuit 2: Lots of cool cars, lots of summer fun. A lot of progress I made on this game was made when I was 9 and it proved on the difficult side to a point. To this day, I haven't unlocked everything in that game, but I have beaten the Ultimate Racer event tree (seemed to be easier running from cops than without them).