These are my current top 10 based on how quickly I would run them.
1. Savage Worlds (SW). I could run or play this game immediately. Some of my best gaming was with SW. I have an ongoing game that genre hops, which has been loads of fun. I expect it to go to the Old West with a Boot Hill module, some Deadlands Reloaded rules, and maybe the setting from Aces & Eights; or to WW2 with Weird War 2. It could go post-apocalyptic (see below). I would even like to try it with low or high fantasy (with an existing idea - see below) or space opera with Star Wars minis. I like the bennies (chips) and cards in addition to the dice so much that I import versions of those rules into other games. It's just so simple to prepare that it becomes much more fun to run and weave the story.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.