Sagat is currently considered the strongest character in SFIV.
Not only has he got the strongest special move in the game (Tiger Knee) along with some ridiculous good normals (standing HK) and a strong zoning game (various Tiger Shots) but his combos mean it takes just one mistake from your opponent for huge chunks to be sliced off your energy bar.
He demands strong execution from the player and has a fairly high learning curve but it's well worth the time and effort spent with him.
High Tiger Shot (quarter-circle towards + P)
Sagat can control the screen using his various Tiger Shots. His High Tiger Shot's main advantage is that it's more likely to catch those starting to jump towards him and can also catch distant jumping players who are starting to come back down to earth. The obvious disadvantage is that they can be crouched under, while some characters can even slide under them, meaning you won't get chip damage.
Low Tiger Shot (quarter-circle towards + K)
Again, these have advantages and disadvantages compared to the High Tiger Shot. They are much easier to jump over but much harder to avoid if the player doesn't want to risk jumping. Usually players will have to crouch and block them, which leads to chip damage, which doesn't do much harm but causes psychological pressure on the other player as it makes them think they are in a weak position while they have to keep blocking Tiger Shots. Good for forcing the other player into taking risks.
Tiger Uppercut (towards, down, down/towards + P)
One of the best anti-airs in the game, Sagat will beat almost every jump in attempt with this move. If you connect with this on the ground, you can Focus Attack Dash Cancel forwards and continue the combo (usually with towards + HK, followed up by something else).
Tiger Knee Crush (towards, down, down/towards + K)
This is hands down the most annoying move in the game. If you space this properly so it only hits once, rather than twice, your opponent won't have enough time to retaliate after blocking the move while you'll be in the perfect position to keep applying pressure. This move also breaks Focus, which means you can pummel away at some characters with this move alone and they might not have the defensive tools to do anything about it.
Tiger Genocide (quarter-circle towards, quarter-circle towards + P)
The lower damage version of Sagat's Tiger Destruction, it has much faster start-up time, so it can punish some moves when blocked that Tiger Destruction can't (Balrog's Rush Punches, Sagat's Tiger Knee if it hits twice, etc).
Tiger Destruction (quarter-circle towards, quarter-circle towards + PPP)
Should be saved exclusively for combos. It connects after a successful Tiger Uppercut that has been Focus Attack Dash Cancelled, a towards + HK when it hits opponents in the air and so on.
Basic Combo: Jump in HK, cr.MK xx Tiger Uppercut
Advanced Combo: cr.LK > cr.LP xx Tiger Uppercut xx FADC > towards + HK > Tiger Destruction