Tsonga and he also has the worst ROS.
Both are true.
Del Potro at times can be the best, but not consistently. Berdych has a big serve, but more usually relies on his ground game and surprisingly good movement for a big guy. Soderling has been out for a while, so I really don't know where he's at.
Tsonga consistently has the best serve.
Most votes got Tsonga- he is smallest one from this group in category ace-per match it is like ciprianned wrote - Tsonga, Soderling, Delpo, Berdych, in category points won on 1st serve it is - Berdych, Tsonga, Delpo,Soderling and in category Service games won - Del Potro, Berdych, Tsonga, Soderling. 2nd serve points won - Berdych, Delpo, Soderling, Tsonga. I think they are very close in quality of serve overall.
Good stats man.