So in a game where you wanted the greatest variabiilty in races/techs... which ones would you choose to play against?
That question could be rephased as "Which civs would you not bother playing against.". There are 12 core civs, and only 10 civs can play at a time (including yourself). Once you knock off the ones who don't add much variety because they feel like another civ, it should be an easier decision to make.
That said, here are the ones who either don't add much, or has a matching pair:
-Drengin/Korath: These two feel very much alike. The big difference between these 2 is the Drengin have a better economy due to cheaper research structures, and because of better economic buildings (standard stuff like stock markets). The Korath give you the impression that they are much stronger due to additional techs and stuff (including spore ships), but their economic buildings are weaker and things like the Aul Incinerator is overrated (+30% industry, reduces PQ by 1, so its like getting 2 +15% industry structures). The Korath do get Dark Influence, as structure that provides a planet with +200% influence (a very big deal).
-Drath/Altarians: The Altarians feel like they were a copy and paste of the Drath. The only notable difference I can think of in terms of tech trees is the techs they get after researching "Xeno Ethics", and the few techs that they Drath gets that relates to their manipulative nature. The Altarians gets many more techs than the Drath after researching Xeno Ethics, including a few that unlocks additional ship modules.
-Korx: They can probally take second place in terms of having the most similiar tech tree to DA, so they don't feel too special. Beyond that, they get a bunch of additional techs for starbases and such, some of which are worth while, like mining modules.
I suppose I haven't quite answered your question yet... only provided a few thoughts to consider. For me, to get the greatest variability means to have the game randomly select opposing civs for me. If I didn't, I might get somewhat biased in my selection of civs. I know that I didn't often play against the Korx and Krynn back in DA.
They didn't seem to draw my attention. Back then, the only difference between civs seemed to be the super abilities, and the Korx and Krynn super abilities didn't seem to cause as much trouble as other civs super abilities did. Some of the notable ones are: Super Warriors (first strike advantage), Super Isolationists (slows down the speed of other civs ships in your territory), and Super Manipulator (reduces the cost of paying other civs to fight each other).