It really depends on what gun you plan to shoot as most holsters are gun specific (with the exception of CR Speed WSM II). If you were shooting a Shadow, I would recommend one of three:
DAA Race Master
USAA Manny Bragg Holster
Ghost Holster
If you didn't want an all our Race Holster, you could go with a Blade Tech.
Here's a little review I have done of the ones I have personally used. We just gut in the Manny Bragg Holsters and they are awesome and have all the best features for a race holster but I haven't used one for competition yet:
DAA Race Master
Pros: Custom fit for every gun means rock solid locking mechanism. Gun does not rock in the holster and you can shake it upside down and it will not fall out. Super smooth draw and fully adjustable. You can change inserts (purchased separately) so you can use it with multiple guns. Metal construction. Great support from DAA customer service.
Cons: You have to purchase an insert for every gun you have. There are not inserts available for every time of gun. Sometimes, the slider that pulls up when you draw the gun drops down the bottom of the holster after a draw and then you cannot reinsert the gun until you move the slider back up. They put a little arm on the holster to allow for this but the first few times, you may not realize what has happened and panic a bit at the range.
CR Speed
Pros: Can be adjusted to fit most guns. It has a muzzle platform that adds some stability. Best holster for scoop draws.
Cons: Because of universal design, the retention is not the best. It feels the least secure of the race holsters. I would not trust hanging my gun upside down in the holster. Locking mechanism will allow you to draw gun when locked if you pull hard enough (some people like that if they forget to unlock but I don't). Draw can bind at times. Plastic construction means you are more likely to strip screws.
Ghost Holster
Pros: Gun specific fitment means guns are securely locked in holster. All metal construction. Uses a metal ball to hold trigger guard for a super smooth release. It is ever so slightly smoother than DAA but does not lock gun in place as well (some movement)
Cons: You have to buy a new holster for every gun.
Bladetech
Pros: Very inexpensive and gun specific fitment. Gun is secure in holster but there is no locking mechanism. Idiot proof design which is great for beginners. You can use these holsters for IDPA while race holsters are no allowed
Cons: You have to buy one for each gun you own. Very few adjustments for angle of draw, spacing from body etc. No locking mechanism