Ok.. well while reading through this thread I changed my mind four times, /Fight, /Ghost, /Ground (Almost went to /Bug here) then back to /Fight.
The reason being that I think Tyranitar is the most urgent Pokemon to effectively check/counter. SS is rampant in CaP, and unlike say Revvy or Stratagem Tyranitar has no solid common counters. It is to variable and the only things that can take virtually all sets tend to be UU/BL or very low OU, because outside of countering Tar they are not that useful.
Also its a great STAB type, SE on Steel/Rock/Normal/Dark and Ice. This means that Tar will fall, Strata can't come in on any STAB attacks, Blissey dies (unless we make it special) and we have another attack against Heatran.
The main problem with it (as Doug rightly points out) is the flying weak. In normal OU Flying is quite a poor offensive type, but in CaP some Pokemon can viably use their Hidden Power slot with it, and things like BounceDDGyrados are not unheard of. That is quite a step. However we can correct this in future by making more Flying resistant Pokemon, that is part of the beauty of CaP.
(next time I'm really banking on an Electric or Steel type though)
Exactly, Electric is LONG overdue. And I suppose Steel could be interesting.
Also to those who fear the grass weak:
Out of those three users of Grass moves in the top 10 (Stratagem: Giga Drain 55%, Zapdos: HP Grass 32%, Pyroak: Grass Knot 33%) only one both commonly uses the move and can do significantly more harm with Grass attacks than other moves.
That means out of the top 10 only one will really use a Grass weakness to its advantage against CaP 6. And even then only just over half of Stratagems carry Giga Drain.
IMO Tyranitar's STAb moves are more important than Grass. Fighting also helps with the ever prevalent SR.
Edit: /Ghost is my second fave type, but the lack of resists and Dark/Ghost weakness is not worth having a STAB move against Rev.