In terms of pure value I'd have to agree with Kram as it pertains to anything that gets you to 1-2 keying NM and UNM Clan Boss.
to take a slightly different approach with champions that will help you in the most areas:
Coldheart: likely a down the line thing as gear improves but Fire Knight and Spider (usually 20-25) all star. Great in some Doom Tower boss fights (Scarab + Fae come to mind.)
Miscreated Monster: A few areas overlapping with Coldheart. Great as the 3 turn shield champion when going first in Scarab. Spider all star due to crowd control and shields. Can do some work in other dungeons if you need a good CC/team protection champ.
Toragi: Reasonable Dragon champ with a pretty nifty kit. However, Toragi is also one of the very best killable Clan Boss champions. Decrease attack A1, ally protect/reflect damage a3 and the passive makes him quite strong for CB.