Isshin because he has 2 legs, 2 arms, 1 head, 1 penis, awesome skill, rides a goat, knows checkers, is funny as hell, cool dude, and he's probably already better than what you got. Honest answer is gain many partners as possible. Look at the stacking bug, that was super fun because of having Drukia and Cute.
Well tbh to add to what nibbles says the way i see it , pvp wise depends on whether you're going for a brute force team where you can wipe the enemy team as fast as possible OR a more strategic / safe approach where you wanna play with your food a lil bit lol.
Yama: THIS MOFO IS DANGEROUS.. Yama obliterates teams.. Yama's niche is just pure destruction. If you have T1 and Yama is your strongest (physical) assaulter it's GG.
Isshin: Plays around with the competitions.. Meaning he just makes the battle longer than Yama in the same gear. He's slower and does less damage than Yama..
With that said.. Ishhin is way better in PVE, where as PVP they are overall similar as a win condition, however I haven't tried isshin in Soul Map yet.. not sure if his skill works there.
It should be noted that Yama gets a mod speed boost first.. of 7%..
edit: i stand corrected Isshin is faster.. by a little.. until first mod yama..
Isshin for sure. Void and combo rate are his bread and butter. if built right (especially with t1), he'll stop your opponent from using their 10th skill along with allowing your main to use his much more often. just throw all your speed gear on him and watch teams crumble before you.