I know this is about Kingmaker but here is a list of pretenders I have had in MP games, played lots, (40'ish? games) and how successful they have been.
Dragons, I have had all 3 colours and about 6 MP in total. I have always had considerable success in MP with a awake dragon. Good beginner choice. I think I have been successful with them because I have taken alot of provinces in the early game with them.
Oracle (astral), about 6+ MP games, I usually take this when I want a big bless, 2 lvl 9's or so. The bless part has always been succesful of course but none of them of ever done really well in the late game.
Ghost King, 1 MP game playing La Ermor, was probably the best ever pretender I have ever had. Great early, mid and end game, was a rainbow and enabled me to get most of the unique artifacts, elemental royalty and tartarians. Of course I was playing La Ermor pre population killing nerf. Won the game!
Ancient Kraken, 5 MPs all water nations, probably THE best awake pretender a water nation can have. Enough said.
Wrym, 2 MPs, awake of course, done very well with, I think I prefer a dragon though.
Great Enchantress, 1 MP, rainbow, awake, successful, was unlucky with manual site searching early game. Still it enabled me to get all 3 air queens, 2 earth kings in RAND playing Sauromatia.
Great Sage, 1 MP with La Argatha, was a sleeping rainbow. Won the game! Again enabled me to literally build every unique artifact (no kidding), I was so powerful managed to battle La Ermor, La Mictlan, La Ctis and La Ulm all at the same time in the endgame.
Lord of Plenty, Ea Tien Chi, was doing well upto mid game when the game was abandoned.
Son of Fenerer, Niefelheim, I was a newbie and did not do very well. He nearly dominion killed another player but then died when he got discovered.
Mother of Rivers, 2+ MP's, good when I want a SC and a lvl 9W bless. Really like this pretender, the free gems are great too.
Forge Lord, 1 MP, Ma Argatha, I was disappointed in this pretender. Did not really help me much.
Well thats all I remember, I have probably forgot some.