Because X-Drake and Urouge were pre-skip the only ones who could stand up against a Pacifista. In his base form he send one flying through a wall, while it took all the SH's combined their ultimate effort to beat one. Drake had the Pacifista in a head lock and was surprised that it had Kizaru's lazers, that is what got him beaten ultimately.
Second, Drake altready went to the New World pre-skip and spend his 2 years there under Kaidou, So I think that is reason enough to believe that pre-sjip he was the strongest, and probably had great training to stay at that level. It's obvious that he will be overtaken by the monster trio of the worst generation obviously. I think Law, Kidd and Luffy will be 1-3 after this arc. This will probably be Kidd's punk hazard.
[video=youtube;7QchLEq6l_w]https://www.youtube.com/watch?v=7QchLEq6l_w[/video]