Best in terms of hardware capabilities for a Nintendo handheld?
Yes, that's fairly obvious.
Best in terms of native software library?
Not by a long shot. Also, it's not very ergonomic (on both the orignal and XL, pointed edges and a circle pad with poor grip that requires constant pressure make my hands ache when playing for extended periods), and the battery life is poor.
As for 3D being included in future Nintendo handhelds, I sure hope it isn't unless they use a totally different (and more efficient) method. The viewing angle is so terrible that I almost never use it, since the tiniest movement I make causes the screen to go dim, and anything more makes the image go wonky. Besides that, there's ghosting, shimmering edge artifacts, and it substantially shortens the already short battery life and cuts the effective resolution in half. I would have much preferred the system just having a higher resolution screen with fewer jaggies on everything. As it is, 3DS's screen resolution is annoyingly low, especially on the XL.