I really think that you'll be better off playing a full game than a set of levels.
The prime reason is that it's easier to focus on just one set of physics and capabilities than that of several games. I find it a good idea to run just one game. I think that once you get "into" the game a little, you'll be able to manage. When using multiple games instead of one, there's also the issue of the game being boring to some players, who might decide to only spend a little bit of time on their run or even skip the round entirely.
I personally think that this competition has a better chance of success if we keep the single-game plan. It won't be as tough as you think unless you're interested in playing it at a
really high level, but I doubt any of you are (and those that are, good luck!)
I think that Rockman and Rockman 2 are the most interesting games, for the simple reason that I've done extensive speedrunning on both of those and am interested in seeing what other people make of it. But probably the coolest game would be Metroid. It isn't as tough as you think. The current world record is at 18 minutes, but I'm positive that decent players will be able to land somewhere in the 21-25 region easily. Metroid is easier because it has more tolerance for mistakes, unlike the Rockman games, which immediately end your attempt the instance you miss a certain jump. They're much more difficult.
So, I voted for Metroid.
EDIT: seems that Rockman 2 is winning. Remember, you can also just speedrun part of the game. In Rockman 2's case, this is easy. Just pick the first few logical stages. The optimal route is Air Man, Quick Man, Flash Man, Metal Man, Bubble Man, Heat Man, Crash Man, Wood Man. The optimal route through the Robot Master rematch is Quick Man, Crash Man, Wood Man, Air Man, Heat Man, Bubble Man, Metal Man, Flash Man. The most difficult part about this game and all other games you're going to speedrun in its entirety is the fact that you need to
stay alive.
I have trouble beating some of those games even when it's not a speed run.
You'd be surprised at how much a difference it can make when you know what you're supposed to do. Watching the current world record videos at the
Speed Demos Archive would be a nice start.