Mass Effect 2 and Oblivion both have combat systems that pretty much consist of "hit the other dude until he keels over". Dragon Age thus wins because even if it isn't the best combat system I've seen, it at least contains some depth and allows for some nifty tactics.
In general, I think Dragon Age is the better of the three games. It isn't as open as Oblivion, but doesn't feel as confined as ME2 either. It has lots of lore, but probably less then ME2 (I can't verify this, but ME2 feels like it has a meatier codex). All in all, Dragon Age is just a good game.