Re: Drogba
A great target man, and fantastic crosser (his accuracy was superior even to the very good RVP), he wasn't clinical enough to be up there with the top three. Case in point, he's taken on 339 shots over the last four seasons (up to March 8, 2012). Of those, he got 149 on target, and 190 off-target, as well as 97 shots blocked. If you compare that with RVP (162 on/160 off), Wayne Rooney (187 on/187 off) and Berbatov (109 on/105 off), you start seeing the problem. Elite strikers get 50% of their shots on target.
His pass completion was actually abysmal (62%), compared to the elite standard (77-80%).
Nonetheless, his 28 assists are good enough for second among all the strikers considered (behind RVP), even if his minutes per goal are a quite-high 235 (RVP=133; WR=150.5; DBerb=159.2).
What you get, essentially, is an elite striker with a few shortcomings that take him out of the running for the Top 3. In most categories he is roughly comparable to the best.
re: Berbatov
He might actually be the second best striker in the lot, if you consider the categories he won, as opposed to the categories Rooney won. Rooney took many of the categories one would normally associate with an attacking midfielder (touches, passes, through balls), whereas Berbatov was very strong in the finishing categories (chance conversion, on/off target ratio, least blocked shots etc).
That Rooney still took the scoring title is testament to his playing time and the number of shots he took on over the course of the last four seasons. Nonetheless, as attackers or forwards go, Rooney is clearly the pick of the litter. He is a lynchpin of United's attack, and should be considered a deep lying forward of the utmost ability.
What surprised me about Berbatov is that, contrary to popular belief, the man is very defensively solid. He doesn't get dribbled past a lot (leads the category), wins the most tackles (% wise) and has both the most interceptions AND the most interceptions / 90 mins. Meaning he closes down defenses and midfielders very well.
In any case, Berbatov's place in the United striking pantheon needs to be reassessed. Scoring at a rate of 159.2 mins per goal means that he is up there with the very best strikers United have ever had... Whatever team gets him will be immensely lucky, his efficiency has actually improved season-on-season.