Top 20 OE actually ain't that crazy, but the jokers are shouting top 10 all time or even top 3 only behind Fedal, which is beyond comical (and I say that as a fan). Not only does the Djokovic FH lack top-end power compared to other top FHs, he can't take it as early as Agassi or even Davydenko, flatten it out or smack it on the run a la Lendl or Pistol, or impart dizzying spin like such dirtmeisters as Bruguera, Guga, Mancini and even Andreev. To put it more bluntly it's the proverbial jack-of-all trades-master-of-none type, which still makes it plenty dangerous but not as much as other ATG FHs.
These guys alone are clearly ahead of Djoko (in chronological order only - DOB listed for comparison):
Borg (1956)
Lendl (1960)
Agassi (1970)
Courier (1970)
Sampras (1971)
Federer (1981)
Nadal (1986)
Del Potro (1988)
Somewhat more debatable but still ahead:
Becker (1967) - now even surer about this after revisiting the Pete-Boris matchup for the 1698547th time
Gonzalez (1980)
That's at least ten. Then you've got the following candidates most of whom these teenyboppers haven't even heard of, let alone have the capacity to evaluate with any technical acumen:
Santana (1938)
Okker (1944)
Newcombe (1944)
Nastase (1946)
Gomez (1960)
Arias (1964)
Gustafsson (1967)
Krickstein (1967)
Korda (1968)
Mancini (1969)
Larsson (1970)
Rosset (1970)
Bruguera (1971)
El Aynaoui (1971)
Ferreira (1971)
Krajicek (1971)
Chang (1972)
Enqvist (1974)
Norman (1976)
Moya (1976)
Philippoussis (1976)
Grosjean (1978)
Ferrero (1980)
Johansson, J. (1982)
Roddick (1982)
Andreev (1983)
Verdasco (1983)
Soderling (1984)
Bautista Agut (1988)
Honorable mentions:
Wilander (1964) - below Novak overall, but 2nd to none in consistency (read: ideal for clay)
Muster (1967) - ditto
Berasategui (1973) - the Hawaiian grip makes it virtually useless on grass, but arguably the most powerful FH ever pound for pound
Kuerten (1976) - likewise the big backswing keeps it from unqualified ATG status, but again what power!
Davydenko (1981) - Novak's is basically a superior version of Davy's FH (though, as noted earlier, the Russian could take the ball earlier with better consistency)
You're welcome to count how many but I'm sure that's more than 20 names. Of course one could argue Djoko is better than these guys and I'd probably agree more often than not, but top 10 all time sounds rather frivolous now, doesn't it? I will, however, add that Novak is superior to overrated ones like Blake, Berdych (though he's somewhat borderline), Monfils, Sock, Thiem, etc.
But throw in historically acknowledged pre-OE masters like Tilden (1893), Johnston (1894), Cochet, Perry (1909), Vines (1911), Kramer (1921), Segura (1921) and Kramer (1921) and you've got well over 30-40. Maybe know what you're talking about before mouthing off? (I know, I know, we're still talking on the internet.)