I'm going to assume you're playing Generation 4 OU, because that's by far the most common format that people play with SS.
Tyranitar is probably the best Pokemon in the game. It has great stats, great ability, and pretty much all the moves that a physical attacker would want. Its all-out attacker movesets can punch big holes into just about any opponent. Pursuit reliably picks off just about any Pokemon weak to dark, and it's among the "certain unalienable rights" according to the American Declaration of Independence.
I'm pretty sure Magnezone is the second best Pokemon you have. (if you can evolve it) Magnezone can trap and defeat just about any steel Pokemon thanks to its ability, defensive typing, and special attack stat.
Tentacruel is useful only if you're sure you need both rapid spin and toxic spikes, AND you need both of them on the same Pokemon. Teams that don't need toxic spikes often prefer Starmie, which has better special attack and can learn thunderbolt.
Mamoswine is good only if you're sure you need ice shard. If you don't need ice shard, then you should probably use Gliscor, because Gliscor has less weaknesses and can learn swords dance, taunt, and roost.
Steelix is good only if you're sure you need a steel Pokemon that's immune to electric. Skarmory is usually better, because it can learn spikes and roost and is immune to ground.
Crobat is useful only if you're sure you need one Pokemon that can kill both sweepers and recovery users. If you don't really need super fang or such high speed, then you should probably use Gengar for the higher special attack.
Ledian's, Fearow's, and Sandslash's stats are pretty low. Other Pokemon like Zapdos or Gliscor are almost always better.
sources: the Smogon analyses, such as this one
