i believe the best unit athrun used was aegis.
there seem to be very little pple giving credit to aegis. aegis's form changing abitlity made it very capable of capturing and trapping opposing MS. its was one of the best used gundams by athrun. too bad he self destroyed it, for a gd cause, if KIRA hasn't miracurously survived that blast, athrun would have the credit to kill the Ultimate Co-ordinator in Strike(the most feared MS at that time)