@ Raebus: He claimed he could, and he was either telling the truth, overestimating himself, or bluffing. I don't see any reason why he would bluff; if he was trying to torment Cloud, he surely would have done it after Cloud had been disarmed; he wouldn't make the mistake of underestimating Cloud twice. Overestimating himself seems more likely, but I doubt it, since he was able to hold back Holy using his will alone, and did control some of the Lifestream.
@ Hyperion4444: Sephiroth and Ultimecia both have to charge those attacks. While they were waiting, Sin would crush them like ants.
And as for Vayne?
Sin 1, Machina 0
@ Wolf Kanno: I agree with you, I was just making sure you didn't write off Sin's power. The problem is, most villains' attacks are similarly untested, so it's difficult to compare them in a one-on-one fight.
@ The Crystal: In Gaia, there is no Kefka, Kuja, Sin etc., so being the most powerful in that world is nothing special.