The JLA would slaughter the Avengers because of their outrageous speed advantage. About half of their core members (the big seven) have insanely high degrees of super-speed--Wonder Woman, Martian Manhunter, Superman, and the Flash--and of their remaining core members, only Batman doesn't have any super-speed. Compare that to the Avengers, who've only had a handful of characters in their entire history with high degrees of super-speed. Superman easily outdoes 616 Thor in speed, but they're roughly about the same in strength and 616 Thor easily outdoes Superman in versatility. Superman can punch, use his cold breath, and emit heat from his eyes; Thor can punch, emit anything from the electromagnetic spectrum from his hammer, call on any weather effect he wants, teleport, and use magic to a limited extent.
The JLA would also slaughter the Ultimates (more easily than they would the Avengers, I'd bet), if that helps put in perspective for you, since your 616 experience is limited.
As for the Sentry, his power level has fluctuated so much in his short existence that it's basically just a big question mark right now. I definitely couldn't even guess at how powerful he is, so maybe he could beat the JLA by himself or maybe he couldn't. No clue.