Can't speak for Delgado (I don't have enough miles on the counter to have seen him riding) but I wouldn't put Sastre and Contador in the same list. I would actually put Sastre and Schleck in the same group, in my opinion they are both climbing specialists (who will both loose a lot of time in a TT).
Sastre was a poor TTer who turned himself into a good one - in fact, he dedicated himself to training very specifically and very hard to improving his performance in time trials, which is something you can never imagine Schleck doing. That level of dedication is perhaps the main factor that separates true GC contenders from the rest. Just look at Wiggins last year.
But maybe, my problem is that I don't quite understand your definition of a GC contender. Since I would put Rodriguez and VDB also in the same group as Schleck. They (unfortunately) are a bit too weak in TT to be a (multiple-)TdF threat.
That's precisely why I mentioned Rodriguez and VDB - both are often talked of as GC contenders but neither have ever looked really likely to fulfil that promise. Yes, I would put them in the same bracket as Schleck for that reason. Rodriguez came very close in the Giro last year but was always going to lose time to Hesjedal in the final time trial. And again in Tirreno-Adriatico on Monday, he was part of the very select attack that won the stage in magnificent fashion but lost his podium place in the final time trial.
Personaly I would put Armstrong, Contador, Ullrich, Indurain, Wiggins,
Evans in a same category (you could make this list sooo much longer of course), which is "all-rounders" (the difference in their results are just depending on exactly how strong their climbing and/or TT is compared to their respective contendors of that time).
Of current riders, I would say there are very few genuine GC contenders - Contador, Froome, Nibali, Wiggins, Hesjedal.
Evans and Basso appear to be over the hill. Gesink and Mollema always flatter to deceive (but are still young enough that they may yet come good). Voeckler ought to be but won't apply himself. Chavanel certainly applies himself but just isn't quite good enough. And then you've got the young upstarts like Vangarderen and Talansky who are thrilling talents but maybe aren't quite there yet...
Anyone else I've forgotten?
[Edit: just remembered Tony Martin, who is the anti-Schleck - phenomenal talent in TTs but always loses time in the big climbs.]
[And Brajkovic - another nearly man.]
I would put people like earlier-mentioned-Sastre, Schleck, Pantani, Rodriguez ... as climbing specialists. Which doesn't mean they can't win the grand tours, but they just have to be climb very strong in the full tour, have the balls to attack from far away and (sometimes) have the brain to decide to focus on the Giro or Vuelta instead of the Tour de France.
Yes, Pantani is another who was never really a genuine GC contender. These riders do win the occasional GT but rarely more than one.
And I guess the other way around will never work: people like Cancellara, Boardman, Martin will always loose too much time in the mountains.
And why Peter Sagan is going to win lots and lots and lots of one-off bike races but
probably never a GT (though I see a lot of Sean Kelly in him and wonder if he might reinvent himself as a GC contender when he hits his 30s - which is still a long way off, let's not forget!)
