Eldar
Pros - You're just better than everyone else, I don't mean more powerful, I mean you're just better. Codex is nicely balanced and there are a number of effective options in each slot. Also the most manoeuvrable army by far and powerful firepower. In my quite biased opinion they are very fun to play.
Cons - Not as weak as Dark Eldar but you still can't stand up to a head on firefight. Model wise they still need a lot of new models, some really, really old stuff still in there (like older than most of the fans)
Necrons
Pros - Codex is ridiculously overpowered, a lot of armies struggle to deal with Cron Air and the AV13 spam. Easy and quick to paint.
Cons - People will hate you if you play them, also Matt Ward fluff. Codex is powerful but there are a few obvious options so there isn't much variety.
Tau
Pros - Nicely balanced codex, very easy to pick a list that can take on anything. A lot of nice models.
Cons - I find they aren't fun to play, for you or your opponent (too much emphasis on being a hard counter to the enemy makes games quite slow imo).