I disagree with earthworm on two points.
Lugia is far superior to skarm. I don't even know how skarm could be the default and lugia a replacement for it. No way man, no way.
And celebi is far superior to miltank. The main thing about ubers is that every kill is going to come from setup or explosion. You don't really have an off turn, nor can you afford to just spend a turn doing nothing. Miltank and other passive mons are openings for anything that sets up. Celebi on the other hand have two phazing moves in seed in song that makes setting up on it essentially a wash. Also plays a part in last poke lax or something. There's just not really any real upsides to miltank at all. You can outpace swords dance, and the other two cursers are both exploding anyway. You get an upper hand vs curse lugias, but that's not worth sucking against everything else.
Mew has its own tier.
Then mewtwo, lugia, and snorlax.
Then the rest of the crew. You really only have 2 slots to play with. There's no real reason not to use those 4.
I guess you can send hooh in vs shadow ball submission mews. Steelix/Ttar are the best explosion takers. I like steelix more tbh, Ttar won't kill anything. Zapdos paras shit, which pisses lugia/mew/mewtwo off. It's probably what makes belling a necessity. Especially mew. Mew is a lot less scary if its slow as shit. That and the burn. Which brings me to celebi, the best beller.
So really, you have like 1 slot to play with. I think celebi/beller is mandatory.