On level of play on clay he's #2 or #3. His level of play on clay is higher than Muster, Bruguera, Lendl, Wilander, and most likely he would have taken out Borg regularly. But level comparisons are complicated by equipment changes. Borg would have been better had he grown up with larger headsizes and used copoly strings, but we'll never know how good. He definitely would not have had the power of Federer or Nadal. He would have been something like a faster, more accurate Ferrer. Would that be good enough to beat Nadal and Fed on clay?
On claycourt record alone Fed is top-10 all-time on clay.
Combining level and record, taking into account that the majority of his claycourt losses have come at the hands of the claycourt GOAT, and he's #3.