Daedroths are lots of fun, but clannfears are probably the best all-around summons for fighting.
I find the low-end daedra to be hard to manage in a fight. Scamps and flame atronachs both prefer their ranged attack, so they tend to back up as the fight, leading the enemy back to your position. Clannfears go immediately forward to engage the enemy, which works best for low-health, unarmored mage types, who dislike being hit. The daedroths are big, and like the big atronachs, are good for blocking up narrow corridors. So they're good shields.
At the lower end, I prefer zombies, who are aggressive enough (and durable enough) to actually fight for you.
Slightly off topic, but my conjuring characters tend to be specialists. My "Fitzie" character only summons undead (she's a Necromancer), while my "Desdaemone" only summoned Daedra like scamps, clannfears, and daedroths (not atronachs or dremoras.)