It's hard to say, as it's hypothetical, but I'm 100% confident that he would walk into that league and be the best player by a country mile. However, it's pointless to discuss these hypothetical things, and when you do then obsessive people dredge them up continually 12 months later.
Messi is never going to leave Barcelona because it's where he grew up, and he's happy there. There is no reason for him to leave.
Also, it is inaccurate and unreasonable to state that Messi continues to play with players that he grew up with. They've virtually all gone now, and today he's playing with Suarez, Coutinho, Dembele, Rakitic, Arthur, Vidal, etc, players he hasn't been around for all that long.
The reason Barcelona are so good is simply because Messi makes them good.
Also, virtually no club on the planet could afford to sign Messi, and no Barcelona president would ever be allowed to let him go, even if bizarrely he did want to leave.
So the argument about him not testing himself in other countries is unfair. However, if Messi did play in Serie A or the Premier League, I'm certain he would be the best player by some distance. It's impossible to say how many goals he would score, but Messi's game isn't about goals, he just happens to score 50 goals per season because he's a genius.
Of course, he can't ever go to Serie A because the only club that would have the slightest chance of buying him would be Juventus, and Ronaldo is not going to allow himself to be completely outshone, obviously. Although it would be a terrible move for Messi in footballing terms because Serie A has never been weaker. I don't think anyone can argue against that.