My usual choices in order:
- Undead: constant menace from lvls 1 thru 15 or so (Necro IV); not much in the high teens, but they show up again in epics / Eveningstar. And since they're SA/crit-immune, all the extra dmg helps.
- Giants: also includes trolls & ogres, so it's a great pick for leveling thru Gianthold; and now that GH is epic-fied, it's become that more useful at endgame.
- Evil Outsiders: mostly for Vale & Amrath; also useful for Threnal, but I've usually outleveled it before I have EO FE.
- Aberrations: grab bag of different monsters - beholders, scorrow, mind flayers, etc.
- Constructs: mostly for House Cannith quests (LoB etc.).
- Elf: mostly for drow in MotU.