Top Tier
Necro is one of the most important dps clickies in the game. It should honestly be in a tier of its own. (Your class is literally balanced around using this clicky as often as it's up .. Use it)
Shadowknight is mini 2.0
Warrior is a mini tank disc.
Worthwhile Tier
Bard increases double attack and (much more importantly) makes you stun immune for 4 ticks.
Mage and Beast are both short duration pet buffs that add a bit of damage and a bit of tankability. Still useful if you're pet tanking.
Monk is a 1 slot beneficial only dispel.
Paladin is a defensive proc that heals you (base 1000, 3% max, so it's not huge, especially in today's game)
Very Questionable Tier
Ranger adds a proc (you'll lose more damage in the .8 second cast time than you'll gain from it, so pre-engage click only)
Rogue increases skill damage taken on mob but only for 10 hits so it's not worth the cast time. Theoretically useful if cast before you would be swinging, I guess.
Zerker's was basically never useful because it increases hit chance by 40% but also has a 10 hit limit. So if cast before engage it is technically useful (albeit barely).
Useless
All the rest have atrophied to literal uselessness.
I think I covered everything there.