I was thinking of learning a Schubert Impromptu, and I was wondering which the best one will be to start with. Any suggestions?

I play all of them. The easiest is No 2 from the second set D935. (Grade 7ish)
For a show off recital piece, try the E flat (No2) from the first set D899. A lot of people like the more gentle G flat (No 3). I also enjoy the F minor one (first set no 4)

Like fsharpminor, I play them all, too, but I learnt Eb major (2nd in first set) first, then F minor one (4th in first set). My favourite ones are : F minor (first one in second set) and Ab (second in second set).

Gb major one is indeed beautiful, too. In fact, I like them so much, that I actually play them at least once a month...

Enjoy learning them!
hero

Thanks fsharpminor and hero I've decided to learn the Eb major one but how to pedal? Help!

For he most part I pedal on the first beat and release on the second. Then in the middle section bars like 85 can be pedalled each crotchet. 165 and 167 I Hold pedal for two bars in each case. 276-279 first and third beats. 280 -281 each beat.

Thank you!