Don't have a top 5, but I really like the D20 System, Mutants & Masterminds, and Mouse Guard (Burning Wheel).

D20's my most familiar system, and my group can easily digest it. Plus it's got a lot of versatility.

M&M is the best system to use for superhero RPGs, bar none. Great for other action genres, too!

Mouse Guard. Haven't had the chance to play it, but I love how it blends game mechanics with descriptive storytelling, and puts the initiative on player action with its unique "turn" system.