CW2 is probably the most purely enjoyable game in PBEM. Though it can also be frustrating if there is a big disparity in AGEOD experience level between players. It can also become very lop-sided in a few turns if NM takes a big turn.
I personally think BOA2:WIA is the best PBEM balanced game AGEOD has made. It's also the simplest, but in some ways that's what makes it so fun.
AJE is really fun in PBEM too, it's a very close second IMO.
RUS is good, but clunky. RUS:GOLD is more balanced, but even clunkier. I do like it though.
I disagree a bit with Durk about ROP, it's a great premise and I've had some fun playing it, but I found the mechanics didn't work as well as other games. I really didn't like the corps mechanics in this game, led to unrealistically large armies. Thought it would've been better with the simple command model of BOA2 (which seems more fitting for 18th century to me anyway).
EAW makes for a pretty good big war experience, though I'm a little biased towards that one
Honestly, they are all fun in their own way, you can't really go wrong. Just pick whichever topic most interests you.