What ysss said.

Although, if you are really into the Nokia N900,
for the majority the answer will be Motorola Milestone/Droid/XT701 due to its similarity in form factor, hardware and software (Android has the closest relation to Maemo, more than OSX, WinCE, Symbian).

But if the question was more of a whats the best phone of another form factor then it would me mixed.
Probably HTC EVO 4G ranking one of the highest due to "specs"
iPhone 3GS also ranking high due to its "simplicity"