The Trio!

[Malrubius]

You have a healer already, and a Shaman is one of the best.

Since you want to do dungeons with a small group, one of you should play an Enchanter. In those tight quarters you are talking about, nothing else can keep everything under control as well.

If you go that route, then the other one should probably be a tank (Shadowknight would be great, but I'm biased) or high-dps+pet class like a Mage.

What you don't get with the above is evacs and rezzes.