It depends on the features you want. If you just want a timer that buzzes and records your time, any will do and lots of suggestions given above.
We sell the CED 7000's and the DAA Watches so I will give you a couple pro's of these:
CED7000:
Very small and loaded with features including adjustable sensitivity, filters, par times, multiple strings etc. They also have accessories like skins, belt clips, battery packs and so on. It features a multi line LCD display.
DAA ShotMaxx
A timer that attaches to your wrist like a watch and has one major advantage. It has a built in sensors that can detect the gun recoiling. Why do you care about this? Well, it means you can shoot right next to someone else at the range and it will ONLY pick up your shots. This is not possible with a microphone only based timer. It also acts like a watch with stop watch, alarms etc. Another cool feature is that it has blue tooth can can communicate with the DAA App for Iphone which allows you to create practice sessions and save all that info to your phone. This timer is also sensitive enough to pick up airsoft and often, dry fire. Pretty cool. This is not ideal for matches as you really can't easily pass a watch around.
I had a PACT IV before and it has tons of features but the thing is HUGE and would slow down as the memory went full.
Everyone uses RU ready timers at matches and they work fine but as mentioned, they only have basic features and no external way to adjust sensitivity and no filters etc.