I use Alesis DM 5 or D4 High Sample Rate Drum Modules because they have 12 trigger inputs and they are easy to program. The sounds are great and the sensitivity of the triggers can be set so that they don't interfere with each other. The dynamic is excellent like on a real e-drum set.
Through the midi connection you can add multiple modules to creat the sounds you like. It don't always have to be drum sounds. How about bass or piano? Or even your midi light system?
Alesis DM5 - High Sample Rate Drum Module
You can also use Yamaha (DTX), Roland, or ddrum modules.
Alesis Drum Modules can be purchased through me.
More product info: Alesis