I too recommend the great Russian composers, especially since you've already mentioned liking Prokofiev. Perhaps start with some of his other works and if you like them, focus on him and maybe a few other composers for a while. Try Rachmaninov too.
Also give some of the Mahler symphonies a listen, particularly the No.2 and No.6, may be on the complex side for beginners but everyone's different.
If you're a newcomer to classical and want to get a good taste then it might be worth using Last.fm, do an artist search for Prokofiev and enjoy what comes up (it will put together a radio station for you based on the artist you searched for). Take note of the composers and pieces you like so you can then investigate them further.