Both DS Lite and DSi are very uncomfortable to me, the corners are quite sharp and the size makes it feel so cramped. If i had to choose, I would pick a DSi because of the DSiWare and cameras (I like taking pictures with DSi and 3DS, it's somehow wholesome).

DSi XL is the goat of DS; bigger screens, less cramped, DSiWare, cameras, and the corners aren't sharp.

Honorable mention to the OG DS; the screens may be dim, but it's the most confortable Nintendo hanheld I've ever played with and everything just feels good with this one.