Not a huge fan of these type of 'best ever' type of debates, but when it comes to goalkeepers during the Premier League era (post-Premiership), I think it's worth participating as we're only talking about roughly 2.5 decades, instead of a complete century or more.
If we're talking EPL era and we're fully objective Peter Schmeichel was probably the best.
• Great shot stopper
• Dominant in the box during set pieces
• Huge presence, both physically (imposing) and mentally (leader)
• Imposing: one of the best one-on-one goalkeepers the game will ever see
• Leader: vocal with teammates, great organiser
• One of the first modern keepers to have great distribution (his long throws were of the highest level)
• Longevity (though, like Seaman did start show signs of regression in his mid 30s)
Schmeichel did have his weaknesses. Similar to Oli Kahn he had a tendency of being hot-headed, which would lead to the odd rush of blood resulting into a subsequent mistake.
Seaman close second. While Schmeichel was more imposing and complete, Seaman was more agile, composed and capable of pulling off instinctive reaction saves Schmeichel could only make in his wildest dreams.
As for third place Cech and Van Der Sar stake a claim. Before his head injury the former was ridiculously good and consistent for a young goalkeeper. After the injury still very good but started to show signs of regression as soon as he hit the age of 30.
Unlike Cech, during the spring of his career, Van Der Sar was nothing more than a solid/steady-eddie goalkeeper, but like a fine wine got better as he aged. One of the all-time best in terms of positioning, reading of the game whilst characteristically maintaining a cool head at the highest level. A true student of his profession.
DdG is still relatively young for a goalkeeper and if he were to stay at United for the remainder of his career he has a strong case of going joint second.
As for most underrated, it has to be Nigel Martyn. Talented, no-nonsense goalkeeper who was unfortunate to have been from the same generation as Seaman; easily the second best English goalkeeper of the nineties.