I love scout, so much fun, I REALLY hate the engineer, I have played him for a while, but then I realized how boring it was, you have two choices as an engineer for play styles, both IMO unbelievably boring, hit and run to your turret, or camp behind it with a dispenser. Meanwhile as a scout it takes some real effort to take down certain classes and requires thinking things through before just running in, or else you die.
No I don't mean it takes the most because that's obviously the spy.
Imo, easiest classes to take out as a scout, Other Scouts, Medics, Engineers unguarded, Heavys, Snipers, and Spys.
Meanwhile the easiest for Engineers to take out are, Spys(Disguised +Sapping) without hiding, Imo.