milestone xt720 is sort of mid range, while galaxy S and Desire are supposed to be high end android devices..

about touch screen, the best is SGS with its real multitouch (up to 5 touch points enabled).. since uve tested it before, u should understand..
I can say that with custom roms and lagfixes, Galaxy S is clearly without a doubt the best fone amongst the 3.. however, if u dun wanna root ur fone and install custom roms, then the desire is better in a sense that it is not as laggy as the galaxy S without any tweaks.. xt720 is also a decent fone, and if 'khalpowers' is here, he will surely push u to get the xt720..

gaming is best on SGS followed by xt720 and then desire..
Best screen is SGS
Biggest RAM is Desire
Biggest internal storage is SGS, followed by xt720 and then desire being the smallest
Best GPS between SGS and Desire, is Desire (not sure abt xt720, that one must ask khalpowers)

If u wanna save money, u can get the xt720, but since ur budget is 2k, i say get either the desire or the SGS, but if u get the SGS, u shud install custom rom and lagfix to have the best experience u can get from the fone..

Best out of box experience is surely Desire. But with tweaks, best would be SGS.

xt720 as according to khalpowers is a really good fone.. But if ur looking to install custom roms, dont go for the xt720 coz apparently it has a locked or encrypted bootloader.. Xt720 is still a pretty good fone without the ability to install custom roms, But personally i would say that if u want to get a motorola, wait for the milestone 2.

Bottom line is, with your budget, get either the SGS or Desire. Depending on ur wants and needs in a fone.