I only use pets in fairly specific instances, and never, ever bother training them.
I have been known to use scrolls of familiar summoning on TF:3 for some easy ACW killers. Great karmic wyrms, titans, ancient karmic dragons, molochs, etc. can all make short work of him. Pretty much anything with fire resistance (needed for TF:4) is probably honestly strong enough to kill the ACW.
Greater earth elementals are common in the very highest DL areas. They're tough, move through walls, and regenerate. Handy if you need some extra help to clear D:50 or Mana Temple (though the Archmage is more than strong enough to kill one of these). Greater fire/air elementals are lame, unfortunately, and spawn equally commonly.
Raccoons and ants are easily tamable with music and will dig rocks for you, which is very nice if you're in need of early game projectiles or gems.
That's pretty much it as far as useful pets go.
Hoping to win with every class, doomed. Archer, Barbarian, Bard, Beastfighter, Druid, Elementalist, Farmer, Fighter, Monk, and ULE Priest down.