Thanks!
I was also thinking 2nd sonata, 4th or 5th. I don't think I would want the toccata at the moment and neither the suggestions lol. Could you comment on differences of those 3 sonatas I mentioned here?
Thank you again for your contribution to my education 
No problem! Glad I helped!
The 2nd sonata is my favorite out of the three, but you can't really go wrong with any of these sonatas. (The complete prok sonatas are on my bucket list to learn...) The 2nd is the least adventurous in the harmonic regard, and really exhibits that signature neo-classical of prokofiev in some of his works. It also exhibits a lot of different characters.
The 4th sonata is a little bit more like the prokofiev that comes later, with more kinds of harmonic experiments. I really like this sonata; it's so grand. You can still hear the references that prokofiev makes the classical era, with its lilting minuet-like rhythm.
The 5th sonata is the sonata composed while the composer was outside of russia. He really starts experimenting harmonies and utilizes other devices like bitonality and such. Definitely the most mature out of the bunch.
I would probably rank them with the 2nd being most difficult, with the 4th and fifth being around the same to each other (depends on who you ask. maybe the fifth is a little more difficult...). Of course, this is just my opinion, and I'm sure many others have differing thoughts
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I would suggest you listen to the short little videos that Bronfman made for Carnegie where he talks about all of the sonatas. Here is the link to the 2nd sonata, and you can navigate to the other sonatas at the bottom of the page:
https://www.carnegiehall.org/bronfman/sonata2/~The Farmboy
Topic: What to learn after Prokofiev sonata 3