Diamond Compressor is really nice for a smoother sound and somehow just makes your tone better. You aren't going to get that tight compressed slap sound if you slap though because it doesn't do enough compression. If you slap a lot then it's not right for you.
The most versatile compressor I've used is the Forest Green Compressor, it's got a lot of sounds in it and they just put out a PCB version that saves you a bunch of money.
Go to Compressor Reviews
for a bunch of compressor reviews. I think if you find something good for bass you will be able to make it work on guitar IMO.