Batman could turn all the worlds machinery against everyone, and eventually branch out. Plus, he could easily make very high powered suits for himself, and bring Brother Eye back up.
Iron Man could almost turn all the machinery against things, but it wouldn't be as cool. Plus hundreds of Iron Man suits.
Flash could rape everyone on Earth before they know it
Superman could really **** some shit up with his Kryptonian tech. As his powers as well, but that's not as interesting. When he had Kryptonian clones with him, they were each tanking multiple JLA members. Imagine if he had utilized the full range among with his newfound knowledge of the Miracle Machine, etc.
Hulk utilizing the Banner tech could be a thousand times worse than Maestro was. A very potential cosmic warlord.
Spider-Man would be interesting if he went rogue seeing as he is quite smart. Maybe not too dangerous, but it'd make for a fun read.
Thor could make it... drizzle I guess? Lightning would be too risky as there's a chance it would hit him and knock him out. He could probably scare small villages into worshipping him
EDIT: I misread (actually didn't read the opening line).
Marvel:
1. Thor - Black Adam. Although that's not so interesting seeing as Adam slaughters him...
2. Spider-Man - Deathstroke. Because he's about on his level, plus his seriousness would make for an interesting contrast for Spider-Man's zany antics.
3. Hulk - Doomsday, since he's pretty much the same person only more mindless and is bony as shit
4. Ironman - Sex Luthor. Genius in a suit. Lex is pretty much Tony with an agenda, with less sex and less booze.
5. Captain America - Captain Nazi. About on the same level, plus Nazi stands for all that is good in Germans, and Cap doesn't even like sourkraut
DC:
1. Superman - Loki. Manipulating magician who likes to **** all sorts of shit up, and call forth many allies.
2. Wonder Woman - Thanos. Because it's funny.
3. Batman - Mephisto. The lord of lies against the darkest knight there is.
4. Hal Jordan - Annihilus. It'd make for some great battles with the Neg zone against the Corps.
5. Flash - Juggernaut... No no, Cassandra Nova. Powerful telepath with a decent shield surrounding her. Somewhat akin to Grodd but on a higher level