Well, I haven't played any of those pieces, but I am familiar with them. I'd say definitely the easiest there is the Fantasie Impromptu. If you're looking for the easiest start there. I think after just finishing Grade 8 the other three will be a stretch for you, as I personally wouldn't be comfortable tackling the left hand of the Revolutionary without proper preparatory exercises and I'm studying for my LTCL. (However, this is probably due to a condition that I have that makes my LH weak, I probably could tackle it with heavy practice.) The Winter Wind is just as difficult if not harder, but the techniques are similar throughout - the same goes for all the Chopin you've suggested really, as long as you can handle the rapid arpeggios and suchlike you should be OK. I presume you're referring to Rachmaninov's transcription of the Rimsky-Korsakov - if you're comfortable with the rapid chromatic scales and the leaps you should be fine, although I wouldn't recommend doing this as it is considered a virtuosic showpiece and after just learning Grade 8, I seriously doubt your technique is developed enough yet to handle this. If you're wanting some Rachmaninov, try some of the op.3 or the op.23 as they're a bit easier and you should be able to manage some if not all of them with practice.
It suggests that you're after some heavy romantic repertoire, so what I did isn't going to suit you as after my Grade 8 I learnt the Mozart K322.(I feel this was more beneficial to me musically than any Rachmaninov or Liszt or Chopin I've ever learnt, by the way.) If I were you I'd maybe look at some of the Debussy preludes for something a bit different, (although not just the easier ones) or some of the Brahms rhapsodies - you will benefit much much more by playing something that you're capable of but stretches you, instead of something very hard that will probably cause you much frustration.
I've tried to be as informative as possible, but if there's anything else I've missed out let me know.