Whomever does it, just do it well: be hype, be semi-knowledgeable with most characters strings/set ups, don't consistently talk over the other commentator, and a very important one in my opinion is if a match is going on don't say anything that you can't get out in one breath; it gives the other commentator opportunities to chime in for the back and forth that makes commentary smooth and also mid match we don't need to be hearing stories about that one thing that happened that one time, with that one player, blah blah blah...and the match is already over before the story finished.
Save those types of stories for in between/after games/matches so you have something interesting and relevant to say when there is dead air/while the next match is being set up so commentators aren't just looking down reading the twitch chat, trying to defend themselves against monsters critiquing their commentary.
UltraDavid does an amazing job whenever he commentates, and as much as people hate on Godspeed for occasionally making cringe worthy puns or jokes, he's also well spoken and understands the game as well as when to talk, when not to talk, and keeps his commentary to the match at hand.
Aris and 16 Bit are also consistently solid on the mic, as well as James Chen even though he doesn't commentate for MKX too often.
