classic was the weakest on paper but he never showed any weaknesses, he was feared by thanos's comrades, he even oneshotted gamora's ship with her in it...killing her (besides mental attack), he was like superman with a one track mind bend on killing thanos.
second incarnation had much more physical strength, basically a flying hulk that could use cosmic energy, his only weakness was his mental deficiency caused by chronus remaking him uninformed of the manner in which he died the first time...brain damage.
his mind wasn't repaired in the re-creation process.
the current incarnation is very different from the other two, as someone put it elsewhere current drax is like current warpath with cosmic energy.he can't even fire it in the manner he used to nor can he fly, but he is a very strong and durable combatant with a conan esqe fighting style.
the strongest version is hard to say because they all have weaknesses that can be exploited.