Rematches between these same pairs as they are now would go like this:
Spider-Man vs. Superboy
This fight ends up being a chase. Spider-man really has very little, if anything, that could truly hurt Superboy at this point. Unless he had access to some kind of knockout gas or other gas as a weapon, he would end up dodging the energy blasts and telekinetic attacks Superboy threw at him until he was finally tagged.
Superboy 8/10.
Elektra vs. Catwoman
As I'm not a reader of either of these, I'll pass.
Superman vs. the Hulk
I think this would go very much like the original fight. Hulk would open up pretty quickly on the Man of Steel, and Superman would have to go all out early on to put the Hulk down. If he took his sweet time, he would end up being in real trouble, though.
Superman 8/10.
Storm vs. Wonder Woman
Storm is no pushover, but all she has is her (rather formidable) weather-control abilities. What it comes down to is Wonder Woman being an extremely aggressive combatant, and once she closes with Storm, there's no question of the outcome.
Wonder Woman 8/10.
Captain America vs. Batman
This one is decided by two things mainly. The first is Batman's wide array of gadgetry, and the other is his willingness to play dirty.
He and Cap are pretty evenly matched mentally and physically, but I'd have to lean slightly in favor of the Dark Knight.
Batman 6/10
The Flash vs. Quicksilver
Yes, this was definitely one of the more ridiculous matchups. It's funny how they tried to add drama to it by having QuickSilver get some shots in while Flash was distracted, and nearly take him down.
The rematch would be no contest.
Flash 10/10.
Silver Surfer vs. Green Lantern
Silver Surfer understands the extent and use of his power much more so than Kyle understands the potential of the ring. A skilled and experienced wielder like Hal would make this fight much more interesting, but I have to give it to Surfer.
Silver Surfer 9/10.
Wolverine vs. Lobo
The second ridiculous matchup. Wolverine gets ripped a new one... and another... and another... and another...
Lobo 10/10.
Namor vs. Aquaman
The way the original fight went was somewhat believable to me. Namor rocked Aquaman in a direct fight, but the underdog pulled it off with the assistance of a whale.
Namor's only real weakness here is his arrogance. The problem is that he's got good reason to be extremely confident. Aquaman is much improved, but he's still not in the same class.
Namor 8/10.
Thor vs. Captain Marvel
This one is likely decided not solely on raw power, but on attitude. Shazam is just too kid-like to pull out all the stops, and Thor is a superior warrior. Plus, that hammer of his tends to make even bigger dents than his fists, which are bad enough.
Thor 7/10.
Robin vs. Jubilee
I haven't seen much of Jubilee lately, so I'll just assume she's marginally improved. Even so, the original fight was about right. Robin is a bit too slick for her, and she'd be taken in by his misdirection and cunning. His utility belt is much more useful overall than her pyrotechnics.
He'd have to make a flawless performance, though, because any kind of slipup would have him seeing stars.
Robin 7/10.
Darkseid vs. Thanos
Both are major players, but I really couldn't say.
Thanos 5/10.