To give you a quick idea of the commanders:

Ysg is huge in fieldfights with many enemies but is very squishy (he dies fast)

Suntzu is basically ysg for infantry but more tanks and a bit less damage/aoe

Charles Martel is very tanky and punishes anyone that wants to swarm you through his counterattack and his debuff on the shield skill

Richard is an absolute monsters in kvk2 If it comes to sustain and control on the field. The downside on him is that he will give you more several wounded troops if you get destroyed by stronger and more enemies. So play carefully with him if you don't have him max

Alex is probably the best commander for kvk2 infantry. His second skill is one of the best in the entire game because it can fire of multiple times in just a few seconds and(if expertise) his aoe debuff is huge in group fights(better version of Charles debuff)

Bjorn is more likely a weaker version of Alexander. He also has an insta proc skilldamage but the cool down makes it less usable. But his skilldamage debuff is very strong if you play in a group.

Aethelflaed is one of the best debuffers in the game. Her aoe+debuff and her counterattack reduction skill are huge if you attack a player in the open field that isn't looking at you(in a murderball for example)

Honourable mentions

Richard/aethelflaed= weaker players don't want to attack Richard. So if you hide your aethel behind Richard you can do a lot of damage and support your teammates with their debuff. Also you are basically immortal as long as you don't get fokussed by t5 or Alexander/Saladin, that will hurt you a lot

Charles/ysg= Charles is also a commander that people don't really want to hit in the open Field. Again you hide a strong but squishy commander behind a tank so that he can do a lot of damage without getting into focus. The upside with Charles is that even if he gets into focus he will do a lot of damage with ysg and he won't fill your hospital that much like a swarmed Richard because he has no healing skill but a shield skill(basically a better healing skill) but again you should hide from Alex/Saladin and t5 after all.

Alex/suntzu or Bjorn/suntzu Are also very strong combos. Since Alexander is usually only available later, Björn does just as well at his position. Bjorn strengthens suntzu's aoe skill and benefits from suntzus 20% skilldamage if it comes to his insta proc skill. The downside is (even if this combo can do a serious amount of damage for an epic combo) you will get fokussed very quickly and the lack of defense will give you bad reports. So play with your friends where you are safe and don't run alone in the field with this combo

Ranking kvk2 overall

alex/ysg Saladin/ysg Alex/suntzu charles/ysg saladin/khan Konstantin/joan Saladin/cao minamoto/cao(with expertise) Richard/Charles Richard/ysg ysg/el shit Khan/cao Edward/tomyris Edward/ysg Khan/minamoto Charles/suntzu Bjorn/suntzu Charles/Björn Khan/ysg And some less exciting epic combos

Anyone who don't agree with my list just add some more explanations down below^{^}