Are you talking about form or overall ability? Because I agree that he's in great form for sure. If not, then Messi, Ronaldo, Neymar, Hazard, Sanchez, Robben, Suarez, Lewandowski, Aguero, Griezmann, Dybala, Bale are a few. He's a good player benefiting from a great system. If Rashford for example played for Monaco, he'd look a hell of a lot better than he has with us. I may be mistaken but most of the goals he's scored in the CL have been pretty standard finishes from chances created by the rest of the team, have they not? I've seen nothing out of the ordinary (worldies, solo runs) that makes him a must buy (apart from his scoring consistency this season). Take him out of the system he plays in and I feel he'd drop a few levels.
As
@prtk0811 says, focusing on grooming Martial into the striker role is a much better idea.