I guess it depends on what you are going to be cutting, and what other machinery you have that needs an extractor. Until very recently I used a SIP 01342 which was fine for general use, although I have recently replaced it with a Camvac 386 twin motor as I wanted finer filtering for MDF dust and more suction for the p/t.
I believe Nilfisk do a number of very good extractors, and they have the advantage of auto switching which is nice to have. However, if you are going to attach extraction to a planer/thicknesser I think you'd be better off with an extractor with a 4" pipe like the Scheppach or Camvac. I have no experience of the Scheppach, but remember that for fine MDF dust you'd need to budget for the fine filter in addition to the standard machine. The Camvac is great, sucks up everything from 0.5 micron dust up to workshop mice and passing children, and has the advantage that the debris doesn't pass through the fan so sucking up small offcuts or screws won't damage it. Costs about £350 if you buy it at one of the shows. But there are others that'll do just fine on a saw, router table, portable power tools (circular saw, Domino etc) for less.