For me it has to be Digisequencer, as that solo is really for the guitar, plus the track is written by Jarre. The Vivaldi track follows a good 2nd place, though not exactly written by Jarre, but its still a very good track to play.
As much as I like Rondat, Im not overly keen on his other performances during Jarre tracks. His solo riff at the start of Rendez-Vous 4 is good, but he can sometimes go for the overkill. I prefere to hear Jarre, not Jarre being slightly drowned out by a guitar player.