Assume lvl 40 pure class or lvl 30 Prestige class.
The only one I've experimented with extensively is the Blackguard Fiendish Servant and Epic Fiendish Servant. A level 30 Blackguard will have a Vrock with 26 levels in fighter as it's servant, and though some of the feats selected are questionable, I've found it quite useful in scroll rich low to mid magic environments. _________________ (I enjoy writing parenthetically (a lot)).
Level 30 Pale Master gets a Demi Lich. Not sure how powerful the default DL is though. _________________ It's rogue, dammit, ROGUE!!!
The PM Demilich is prolly the best - level 21 Wizard, immune to level 3 and lower spells, undead properties and DR (I'm not sure how much), but taking that many PM levels kills your caster level. The epic Shadowlord has some nice immunities, but no concealment unfortunately. None of the epic summons are really worth taking that many levels in one of those PrC's IMHO, you just lose out too much if you do. _________________ "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices, but honestly and courageously uses his intelligence" - Albert Einstein
I'm not sure either, but I can tell you this, a full level Animal Companion (I think it's level 43) with Greater Magic Fang, Bull's Strength and Awaken is an awesome ally, especially the panther if you help it land sneak attacks (like stunning enemies with Storm of Vengeance or prone with Knockdown), it gets hell powerful.
I prefer Animal Companion over any other summon because you simply feed the little guy and he's ready to roll, plus, he's there for you at any level. The only real drawback is his awfully low AC.
The PM Demilich is prolly the best - level 21 Wizard, immune to level 3 and lower spells, undead properties and DR (I'm not sure how much), but taking that many PM levels kills your caster level. The epic Shadowlord has some nice immunities, but no concealment unfortunately. None of the epic summons are really worth taking that many levels in one of those PrC's IMHO, you just lose out too much if you do.