Personally, I recommend the CyberVision 3d/ ScanDoubler combo.
Why? The integration.
With the scandoubler, you also get a monitor switch built-in. You can Amiga-M (filp screens) between a CyberGraphX screen and an ECS screen like they were both being generated from the same source. No manual switches, no hassle. Nice touch. Multiple monitor support is also available, if you'd rather go that route... Plus, you can use a VGA (or better) monitor for all Amiga generated screens, as well. Probably not a huge selling point, as you can already do that with your 3000, but it was a large seller for my A4000!
It's also one of the better supported cards, as both CyberGraphX (all versions 2+) and Picasso 96 support it.
And, finally, it has reasonable performance / price. Only a few ticks slower than the original (scarce!) CV64, and a lot cheaper than buying a Mediator solution. (Which I haven't seen a real head-to-head power comparison on non-PPC machines... Somehow, I'd tend to think that a 680x0 might not give the Mediator/Voodoo combo much of an edge, either...)
Fair disclosure - the only two cards I've ever owned are the CV64-3d and the Retina Z2, and I haven't seen the performance stats on the Mediator/Voodoo on non-PPC.
I can say the CV64-3d is the best integrated and most finished product I've seen, though. It drags the RetinaEMU bench out behind the shed and beats it.
