Only been hunting for 5 years, (and learning mostly on my own) but of all the things I've done...
Turkey is the best, above deer and everything else. That feeling you get when you hear (and if they're close enough, almost feel) that gobble is unmatched. Audible feedback on whether or not you're doing OK is nice too!
Squirrel is nice because you don't have to take it super seriously (but you can if you want to). It can be a stroll in the woods with a gun, or you can put as much effort into it as deer. I like to grab my .22 after work, throw some camo and orange over my work clothes, and take a stroll in the small state game lands nearby. It's a nice way to clear your mind after work, and get some dinner too. I don't enjoy cleaning squirrels though. The effort:meat ratio isn't great.
Waterfowl, I'd really like to get into (like turkey, I love learning to call), but I don't have any friends that are interested, and I think it just seems like something that would be more fun with friends. I've gotten my duck stamp every year, just don't really head out. I did get one bird, a banded Canada Goose that I got from my kayak an hour before work (can't say I've ever changed from waders to business casual clothing before). I thought it tasted pretty darn good, not sure where goose's reputation for tasting bad came from.