My previous favorite was the Natural 4000 Ergonomic, and now the Sculpt Ergonomic. In both cases, I have had keys wear out* and have gone through quite a few, but like them enough to put up with the trouble. Of course, I could get the 4000 for $20; I tend to have to spend closer to $60 to get the Sculpt!
* The Natural 4000 space bar was the worst offender. On my Sculpt Ergonomic, I've lost an 'L' and 'F5'.