I've been ok with Norelco and Braun. I've used cheap and midpriced models, no super expensive ones. I'm unpersuaded that expensive ones have real benefits, but who knows.
As with cordless tools and ipods, one thing I hate about cordless razors is internal rechargeable batteries. I see them as a scam designed to get you to throw away the expensive razor once the batteries stop holding a charge. I've replaced them a few times (take razor apart, order solder tabbed AA nicad cells from someplace, unsolder old cells, solder in new ones, put razor back together) but it's a pain in the neck and I had a Norelco refuse to start up after the 2nd or 3rd battery change.
Cordless has some convenience but I'd say make sure to get one that is also usable from corded power regardless of the battery state. There are a lot where the AC plug only recharges the batteries, which means if the batteries are discharged, you can't shave until you've left the thing plugged in for a while to recharge. That's a big nuisance
Right now I'm using a Norelco (I can check the model number if you really want to know it) that is completely sealed, so you can rinse the whole thing under the faucet after using it. That sounds like another gimmick to make sure you don't replace the batteries yourself, so I'd have resisted, but I got the thing cheap and the feature turns out to be very convenient. I'll figure out what to do about replacing the batteries when the time comes.