[ArkhanTheBlack]

I make a big difference between commander summons (including non commanders which get all equipment slots with GOR) and monster summons.
I think there are a lot of good commander summons on different research levels, my favourites being the sea kings court, Wraith Lord, Bane Lord and the arch devils. I have an exceptional dislike for Tartarian Gate. Regardless what you get, 10 death gems are just absurdly cheap! You don't even get a bane lord for that.
There are also some very powerfull race specific summons, especially from Agartha or the Ulm Vampire Counts. There are probably more, but I didn't play all races yet.

I personally think most monster summons are highly overpriced or I didn't find the right way to play them yet. The only good ones are Tarasque (first) and Abomination (second). However, both are overshadowed by the ridicilous cheap Tartarians. Leviathan also isn't that bad if you're playing a sea nation.

I also like the Pangaea Carrion summons...