These 4 champions are very useful, as they have versatile build paths, and can carry games pretty hard, however, it is usually pretty important to have at least 1 ap champion in your top lane pool so you don't have cry as you're matched against a malphite.
IMO, the best AP tops are
: INSANE utility, with a snare, slow, long ranged gapcloser, invulnerability, suppress, and reliable poke. She's basically built for the top lane, since she doesn't scale as well as other ap mids, but has high base damages and strong teamfight presence.
: Pretty similar to lissandra, but is tankier in general, with more sustain. He also brings a lot to the table, with some damage reduction, a gap closer, a small knockback, a big knockback, and some zone control. He's pretty easy to learn, and hitting a perfect ult in a teamfight is such an amazing feeling.
: I pick swayne into matchups like jax and irelia, because he has huge potential to bully early, but still becomes a god late game. While he lacks the sheer amount of utility of the aforemenioned, his damage and tankiness make him a solid pick.
: This guy will make the enemy laner's life hell. With his huge zoning potential, near invincibility in lane vs. melee champs, and a strong ability to be ganked and pick up a double kill, this guys almost as bad as teemo.
My favorite ap top laner, this guy has strong sustain, wave clear, consistend dps, and insane burst damage early game. He scales well as a full tank, a hybrid, or burst ap, and provides excellent objective control as well as strong teamfight presence.