I liked DS and managed to get through it though I was forever putting off Ai's story Ai please stop screaming at me, I started with it but then recently got SP. The gameplay is really different, to the point I think I may have cocked up my SP playthrough with the DS mindset. Lesson learnt. orz
I've never played any of the Xbox games, but if you manage to start with a portable game I'd make it an SP one first.
Hmm...well, as far as the iM@S games where you play as Producer, SP
is the one I'd recommend starting with. You learn how to play the game well because it's easy

iM@S1 is a lot harder than SP, and then iM@S2 is...well, in the middle, I'd say
Though, I never had a problem producing in iM@S1, and my first iM@S2 run was a bad end XDD.
In general, even though the games would be out of order (not that it matters, none of them are connected anyway), I'd recommend going from SP to iM@S1 to iM@S2.
..and, I guess, throw in DS wherever, if you want to give it a go.