OK, I somewhat obsessively crunched some more numbers and put a bunch of players through the same formula that I used to rank players in my nation series. It is more detailed, including all Slam results (from Wins to 2R), titles, and year-end rankings. Here is the list, with the point totals for comparison's sake:
1. Ferrer 390
2. Okker ~350*
3. Davydenko 311
4. Berdych 272
5. Gottfried 270
6. Rios 258
7. Nalbandian 257
8. Solomon 256
9. Corretja 253
10. Tsonga 252
11. Henman 248
12. Haas 246
13. Enqvist 231
14. Robredo 225
15. Ramirez 224
16. Martin 222
17. Mecir 210
.... Medvedev 192
.... Philippoussis 178
.... Soderling 174
*incomplete data; a lot of estimation
A couple things. One, I'm fairly certain that 1-17 is complete, but my guess is that there are players that slot in after Mecir and before Soderling.
Two, as you can see, like the simpler system, this one likes longevity over players with short but excellent careers, like Miroslav Mecir (who might have had the highest peak of any player on this list).
Finally, I think it is interesting how clumped together most players are, at least from Berdych down to around Mecir, but especially in the #6-12 range. I think with some error in the system and the fact that it doesn't take into account everything, you could slide people around within those ranges.
But in conclusion, I think it is clear that here are three real contenders for best player never to win a Slam: Ferrer, Okker, and Davydenko. Ferrer and Davydenko are similar - very consistent players with tremendous longevity, but not amazing peaks. Okker is from a different era, playing from the mid-60s to early 80s and thus played during a time when a lot of players skipped the Australian Open and French Open. I think if he had played more of those tournaments his overall number would be equal or higher than Ferrer's.
My vote goes to Tom Okker, with David Ferrer a close second.