There are 4 threats that are above the rest: Tyranitar, Suicune, Snorlax and Celebi.
Tyranitar, as you said, is the king of OU. Eternal sandstorm, great stats, it can be a great hard hitter, a wall breaker, the best late game sweeper of the game (DDtar is a monster).
Suicune is really hard to beat if played well. The defensive version is really hard to beat (especially with pressure wasting PPs), but the really dangerous version (on this metagame) is the offensive one, especially with Sub. This monster can set up on things like Bliss, it can beat defensive Celebi (most of them), Defensive Zapdos and most of the common Suicune switchings. Things like offensive CM Celebi or Superachi can beat this, but ironically, a defensive set with CM + roar will destroy them (and nearly every cm sweeper other than raikou).
While insanely sturdy on paper, in practice...well it's still pretty good, but its survivability is cut down in recent times by having to take repeated attacks from Aerodactyl and Tyranitar in Sandstorm. I wouldn't mind pushing this up to 8 or 9 if I weren't so lazy about it.
It can take 3 CB aero rock slides (and aero only has 8 rock slides vs pressure suicune). Sand + Spikes can be annoying, but thats true for so many things.
If a pokemon can destroy stall (the Subcm set can easily destroy standard stall), and it can wall and destroy a lot of sweepers (it can take 3 CB HP flying from a Salamence, which is pretty impressive), it definitely deserves being much higher.
Celebi, the pokemon that never dies. It can be a great support pokemon for a stall team (heal bell, reflect, perish song...), it can be one of the best bulky sweepers of the game with CM + leech seed, and it's really annoying to face. If you give it a free turn for whatever reason, it might pass a Swords Dance or a CM to some sweeper, and that could be gg for you. And it can also use an offensive CM set, that can easily destroy some of the common bulky CM set counters.
I think you are underrating Snorlax a lot. It's not as good on this metagame as it used to be, but it's still the best last pokemon sweeper of the game (maybe with the exception of crocune), it could even surprise Skarm with a mixed set with Fire Blast, or even explode by surprise. And you can never forget the curse set (not nearly as good as gsc lax though), capable of sweeping any team with the right support or if used on the right moment. Yes, I agree, it's much worse on this metagame, but I think it deserves a place on that top 10 list.