Water is probably most often dropped, since it doesn't have anything that irreplaceable to it other than being able to abuse dwarves' only vuln and protection vs. cranial.
Other than that you just have to choose what you're willing to give up and who you'll allow yourself to be weaker against.
Air has improved invis, which lets you abuse lower-level people with impunity. Buffet does nice damage against fliers. Vortex can be funny. Shield of winds helps against getting trip-ambushed and helps against assassin kicks.
Earth has dig for concealment and trapping. Pebble can be used for ganking, rain of stone is also funny. Earth ripple can be handy. Shield of earth gives resist bash and protects vs. bearcharge.
Ice has some really heavy single-target damage spells in iceshards and iceneedles, cone of cold hurts too. Shield of ice gives resist slash and protects vs. pincer.
Fire has immolation which can be nice. Controlled fireball deals out respectable damage, nova is a great ranking spell. Shield of flames stops/hinders some contact moves like blackjack and tiger claw.
Lightning is all about damage and in many ways similar to fire. Pillar is like an outdoors multitarget nova with a chance for residual damage. Forked lightning is like controlled fireball. Shield of electricity can counterattack metal weapon hits with massive damage.
Ooze has about all your "tricks", with grease, adhesive web and good old quicksand. Vitriolic stream works if you're being lagged, which can be handy at times I guess. Shield of slime helps against lash and entwine.
Personally, I'd never give up air, ice and ooze.
Edited 1 time(s). Last edit at 07/10/2013 11:41AM by Mort.