Here are the top 10 players with the most career titles on clay:
1 Guillermo Vilas 46
2 Thomas Muster 40
3 Ilie Nastase 31
4 Bjorn Borg 30
5 Manuel Orantes 29
6 Ivan Lendl 28
7 Rafael Nadal 25
8 Jose-Luis Clerc 21
9 Mats Wilander 20
10 Andres Gomez 17
With the French Open title, Federer now has 9 clay court titles. 4 straight French Open finals with one championship is impressive, and we'll see where Federer ends up by the time he retires. However, calling him a top 5 clay court player ever is incorrect at this point. In my opinion, the top 5 is:
1. Borg (6 French titles, 30 overall clay titles).
2. Nadal (4 French titles, 25 overall clay titles, so far).
3. Lendl (3 French titles in 5 finals, 28 overall clay titles.)
4. Wilander (3 French titles in 5 finals, 20 overall clay titles.)
5. Vilas (1 French title in 4 finals, 1 US Open on clay, 46 overall clay titles.)
Guys like Kuerten (3 French, 14 overall clay titles), Nastase (1 French, 1 US Open on clay, 31 overall clay titles), Bruguera (2 French, 13 overall clay titles), Courier (2 French in 3 finals, 5 overall titles), and Muster (1 French, 40 overall clay titles) could either round out the top 10, or be argued into the top 5.
In order for Federer to break into the top 5 to 10 clay court players in the Open era, he needs to win the French Open at least one more time and start collecting more clay court tournament trophies. With another 5 to 7 years on tour (which he says he wants to do), there is definitely a chance that he could scale into this category... but he's not in the top 5 (or 3) clay court players of the Open era right now.