A couple thoughts (caveat - I have no dog in this fight).
(1) "directly" better vs. better overall. Here's where a lot of the debate lies. Ferrer has winning H2H records vs Tsonga, Berdych, Del Potro. I'd say the 2-1 lead over Tsonga is negligible (few matches, Tsonga won only Slam match), but the 6-3 record vs. Berdych, and 6-2 record vs. Del Potro (including the last 4 and the only 2 slam matches) is substantial.
My point isn't that direct H2H doesn't matter in a "better" analysis, it absolutely does. But, it's only one factor. "Better" generally means "better overall" - where a variety of factors are looked at. If it were only H2H - then James Blake, when he briefly had a 3-1 or 3-2 lead over Nadal in his H2H would have been "better", but that sounds ridiculous.
And what are those "other" factors? The things we talk about all the time - overall titles, Masters titles, Slam wins, other significant Slam performances, rankings, etc. And, as always, we can all disagree on how each factor should be weighted. Outside of his one Slam win, Del Potro has the weakest Slam resume of the 4 players, but he has a Slam win.
Looking at other factors, there's a very strong argument that Ferrer is indeed "better overall", but, again, some people understandably place enormous value on a Slam win, almost to the exclusion of everything else.
Ferrer - 20 titles overall (including one Masters), 5 Slam SF, 5 Slam QF, High Rank No. 4, Davis Cup titles
Tsonga - 10 titles overall (including one Masters), 1 Slam RU, 3 Slam SF, 5 Slam QFs, 1 YEC RU, high rank No. 5.
Berdych - 8 titles (including 1 Masters), 1 Slam RU, 2 Slam SF, 4 Slam QF, high rank No. 6, Davis Cup title.
Del Po - 14 titles (no Masters), 1 Slam win, 1 SF, 5 QF, high rank No. 4.
Again, who is better overall depends on how much value you place indivdual factors. Del Po has the worst h2H record vs Ferrer (2-6) of the three players, but he has the Slam win. It can be debated endlessly who history will consider the "better" player, and more likely, history won't have a definitive answer.
(2) Top level of play vs. general level of play.
This is another area that provides a foundation for the "better" debate. The stats show that Ferrer's level of play is generally better. But, some people understandably focus less on general level of play (not ignoring it completely, but just focusing less on it), and more on peak level. And, many people conclude that Tsonga, Berdych and Del Po peak level is higher and "better" than Ferrer, and that they pose more of a threat to the Big 4 when "on" than Ferrer generally does. Would Ferrer come back from 2 sets down at Wimbledon to beat Fed, would he come within a point of beating Djokovic at the French Open, would he beat Fed at the French Open or the USO - probably not.
I do beleive he isn't as much of a threat to the Big 4 on any given day as an "on" Tsonga or Berdych (not sure about Del Po at this point), but looking back at all of their resumes at how many times they beat Big 4 at a Slam, isn't it all fairly similar. Tsonga has beat all 4 at one time or another. Delpo beat Nadal and Fed in the same tournament. Berdych has beat Fed a couple of times. But, Ferrer himself has beat Nadal twice and Murray once.
Tsonga is the one guy I still think whose "on" game against the Big 4 is always going to be more of a threat than Ferrer, but if a "threat" doesn't end in a win, does it even matter in debating who is better?
So, I'm done with my mini-essay. I think you can easily argue Ferrer is better, but can understand why some people won't reflexively anoint him "better" than the others, or at least some of the others.
