I will write only for male players, female gets too complicated by default (since we have to take into consideration who has a normal cycle or not, if they are in medication for not having a cycle, etc etc).
1. David Ferrer (credit to him though for being for so long consistently in the top 10, even top5)
2. Tsonga (these athletic skills and so poor results, really pitty, he needed support from a different coaching team)
3. Monfils (pitty, makes things too complicated)
4. Andy Murray (for his hard work, and so skillful coaches around him (I mean his old time true scotish team) he brought very poor results, he should have done better against Federer at this age, Nadal with his injuries, Djokovic with his tandrums. He spends too much time reading the british press, and commenting "politically correct" crap. I wish he had Lendl 10 years earlier and man him up sooner.
5. Kei Nishikori & Dimitrov (would expect more from both)
Federer and Nadal are certainly some of the All Time Best but afterall these 6 ATHLETIC guys should have done better. I mean if they continue like they do now, they kill keep loosing from Federer and Nadal even when Federer reaches 50 years old.
That does not underestimate Federer if he beats them when he is 50, but definetely underestimate these 6 guys.