I've really enjoyed the Megaman Z series, especially with the ranking system. They're all pretty short if you don't care for ranks or unlockable items and such, but trying to get the perfect score is insanely hard and gives a great feeling of achievement. You get a totally different view of the levels when trying to go through at maximum speed, killing EVERYTHING and without taking any damage yourself. Kinda like doing a perfect speedrun.
I think every game should have this kind of speedrun/ranking system. Ninja Gaiden for example is a new game each time you aim for a higher rank, and again renewed when you start on a harder difficulty level. At the point where you really have to push Ryu to his limits it really starts to feel like you're a proper super ninja.
Then games like Contra are just plain hardcore and not so easy to access, so the system in Megaman Z gives a good game that's easy to access, as well as something for the totally hardcore people. I think today that games HAVE TO be more and more accessible to cover for rising developement costs, they sometimes forget smaller audiences that just don't get turned on by something too simple. This is why having something more deep hidden under an accessible shell is just perfect. Not to scare away too much, but also to give more if you so wish.
One time me and my friend were having a competition on who completes the first world of super mario world the fastest and it was so much fun.
Oh and long story short:
Ranking systems
As for Phantom Hourglass... want it NAO! Maybe should consider importing from US when it's released, since DS is region free and sometimes the games are cheaper as well.