I've gotten a lot of enjoyment watching (and participating when I can) in the development efforts to produce an alternative OS for the N900. Last June, it seemed activity had organized around 2 main ideas.
If we learn from the n800 experience the answer is none. The older tablets were too great devices just how the n900 now is. An it will be (mostly) forgotten as well when the next thing comes along. Get used to it, it's the speed of this business. So I say maemo will be the best just as maemo diablo still is the best os for the older tablets.