dbx 163x or 160a
if you can find one cheap. The mods to the 163x clean it up (you can DIY it yourself as well) and even stock it kicks ass on vocals. We use a modded one daily at Studio E, does high levels of gain reduction and still sounds smooth and full. Works on everything from RnB vocals, rap vocals, to kick drums and basslines. I truly love mine.
I recall the 166 (no X) was supposed to be good as well.
I also recommend the Yamaha GC2020. Highly poo pooed, maligned and shot down in the live sound world, its actually a well put together product.
One day when I was feeling like gettin my geek on, I opened mine up and found that it had Rubycon capacitors all over the place. These are the capacitors that people tend to UPGRADE TO. So I was kind of shocked to see the Yamaha "cheapo," compressor have them on the PCB, but internet griping can't fight real life experience.
If you wanted to mod it, I suppose you can change out the vcas and opamps to see if that improves the character or the sound but to be honest its a good unit to have, it works for vocals and drums. Its actually pretty quiet considering the pedigree it inherited here on Gearslutz.
I actually will use mine TMW on a high profile mix and see how it fares.
The GC2020 can be found for as cheap as $20. Sleeper for sure.