Best in the world right now, but still below Schmeichel level overall for me (as far as United keepers go). In terms of wider performances over the last 25-ish seasons, I'd rank him below only the quartet of Buffon, Schmeichel, Kahn and Neuer (going through a mediocre patch by his standards, but his overall peak was phenomenal) - with one on par with De Gea (mid 2000s Čech - who was a different beast pre-Reading and gets severely underrated), and just ahead of the likes of Preud'Homme and Van der Sar and Peruzzi. But the thing is, it's hard to tell at times from an individual standpoint - and you can't quite put him in context because they all had good to great defenses in front of them (with maybe an exception or two here and there), and performed in big, big European and international matches. Though De Gea does still have a lot of time on his side, so he could end up really close to, or level with a couple of the the Top 4 (and that's no minor feat because all of them are in, or right on the edge of Top 10 all time status). And he is probably the best pure goalkeeper from that entire selection apart from only Buffon (who's the best pure goalkeeper ever, IMO), and probably Kahn.
He is very unlucky in that his emergence has coincided with United going through a rough patch (which again, makes it hard to evaluate his performances from a micro and macro perspective in big European matches - something that Buffon, Schmeichel, Kahn, Neuer excelled at - and in comparison with them, David has played maybe 4 big European matches overall - vs Real Madrid and Bayern Munich), as well as the Spanish national team going through a rough patch after an era of overwhelming success which inflated Casillas' overall stature - despite him being inferior to De Gea as far as goalkeeping pedigree goes. Put him in place of Casillas on those teams, and De Gea would be talked of as one of the genuine all time greats, and if he had maintained this level - then very close to Buffon's GOAT class. David really deserves a better platform to showcase his skill, because he really isn't lacking in terms of individual talent. Sure hope we can provide him with that, or he won't stay for long (and you won't be able to begrudge him, because not playing in the Champions League regularly doesn't suit a keeper of his standing).