To be honest after watching this video it isn't even a question who is better.
By the way, remember Adler? Was regularly linked to us with the nearing VDS exit, our fan and Germany's number 1 before Neuer. You can go wrong with German keepers.
Don't think that Leno or Ter Stegen, let alone others, are capable of what De Gea produces over the last few years.
There is always Donnarumma, of course, although it's hard to predict his career trajectory at this point.
Although Donnarumma is an extraordinary talent but trying to get a young player like that with the pressure he'll most likely have is not a good move in my opinion - for both parties. Donnarumma would benefit to stay in Italy for a couple of years, since he's still very new.
Well of course Neuer is better but his qualities are unique, no one's near to be as good as him when it comes to his best "features". Honestly, I don't really think there are a lot of good replacements, which is why I'd hope he'll stay. Leno is basically the only GK with the right experience, age, height and strengths that I can think of at the moment. Feel free to name better choices, not that easy.
I would almost prefer it if Romero could try to be the no 1 if De Gea leaves since I think he's done a good job when he's got the chance. Real Madrid could probably include Keylor Navas in the deal but would Navas be interested in playing in the PL? De Gea's replacement in Atletico Madrid, Oblak, is good but isn't he a bit too short?
Like I said, hope he De Gea stays, because there aren't that many great ones out there. The same as the type of striker Ibra is - the type Mourinho likes.