It's really hard to say who is the best boxer in MMA considering all the variables they have to factor in which can totally hinder their actual boxing skills. Ie., the threat of a takedown, catching a front kick to the face, spinning back or side kick to the body, kicks to the kneecap, leg kicks etc just to name the most obvious ones. Alex is up there for sure though, I'd say McGregor definitely is too (turns out he was at least a Junior National champ in Ireland after all). Mousasi is in this discussion as well.
For what it's worth, Alliance Gym's head coach Eric Del Fierro seems to agree with you, TS. He's Alex's
now former coach when he trained for a few weeks at Alliance leading up to his fights. This guy knows a lot about boxing and striking, in fact, he's definitely a top 5 (I'd even say top 3) MMA striking coach. What makes this impressive, to me anyway, is that:
1) The guy is honest...
so honest that he even told the media that he had Gustafsson actually LOSING (though close) against Jones during the fight - up until he watched the tape after the fight and changed his mind, scoring it for Gustafsson.
2) He's not even Alex's full time coach, eliminating at least 'some' bias from him. In fact, he's a very part time coach for Alex, only coaching him leading up to his fights for a few weeks. Andreas Michael is Alex's actual head coach (and boxing coach), he's with him at all times both in the states and back in Sweden where they both live. Andreas Michael himself is a former Olympic boxing coach and clearly this man is responsible for Alex's ever-improving boxing game.
3) Del Fierro coaches guys like BW champ Dominick Cruz (a very solid MMA boxer himself) and Ross Pearson (very solid MMA boxer too). Both of these guys are in the Top 15/20 most talented 'MMA boxers' as well.
Del Fierro:
"His boxing is bar none probably the best in the sport right now."
Source: http://www.sherdog.com/news/news/Trainer-Alexander-Gustafsson-Wouldnt-Let-Jon-Jones-Set-Up-Takedowns-56897
Del Fierro saying "bar none" and then following it up immediately with "probably", however, is open to interpretation. It sounds more to me like he didn't actually mean "bar none" and softened it a bit with "probably". Still, it's a great compliment from an amazing striking coach who has seen more than enough of Alex's boxing game over the years at Alliance to assess it and form his own opinion.