(in no particular order) I don't know dick about ominverse aliens since I didn't watch it so I'm not putting them in
Water Hazard(?) I thought he was overflow but, I guess not
Mainly because I like his design and water powers are cool.
Big Chill (not whatever the fuck omniverse abomination that is in the pic but OG one) He can phase through shit like ghost freak BUT CAN USE ICE.
Echo Echo I like the idea of splitting yourself up and basically spending the whole match fucking with your opponent with dupes of yourself.
Lodestar I like his design and voice.
Swamp Fire (wish he didn't look so retarded except when he looks like a tree with blue napalm in him, that's cool.) Trapping enemies then inflicting them with burn is pretty neat.
Wildmutt Decent enough tracker if you need to find anyone, yes you are basically a retarded looking dog, but you can easily time out of the omnitrix after you find the dude and switch into someone more useful if the situation calls for it.
Ghost Freak I just like his design. I forgot if he's like that one wolf motherfucker that you can't control, but if you can then he's pretty solid for a stealth alien plus if you're going after some weak normalfags he'd instantly be used for scaring the shit out of them.
Spidermonkey I just like his monkey voice.
Chromastone He can fire pretty gem rainbows... of justice.
Grey Matter Small but, you could easily build yourself a cool weapon or mech to fight people with, that could come in handy when the omnitrix times out.
Alien X He'd be the trump card that would curbstomp the enemy if shit got bad. (Inb4 you gotta listen to arguing; I'd probably try to talk to them for fun on weekends and only ask for their help in really dire situations. "Promise guys, this is the last time.")
I know my sets shit but, I mainly picked one's I liked rather than how OP or useful they were in the show.