For a leather class I'd either pick Druid or Monk (well, I personally wouldn't pick druid but yea)
Reason being, it's really hard to get bored as you can do all three roles, you can DPS, Heal or Tank if you want, fit into most patch changes and switch playstyles to make the gameplay feel refreshing.
Last leather class is a Rogue, and I don't think they're interesting at the moment, but as a strong believer in making a character for life, not for a patch, if you like rogue playstyle - go for it. It's a big change from any other class and can provide lots of fun.