I just started my Empire Strikes Back Boba, because i think (as almost everybody here) that ESB is just amazing.
The first time i saw Boba Fett was in ESB, so much mystery and deadly attitude, we could feel his presence just as Vader... I wouldn't say the same about Boba in ROTJ, which to me appears to be kind of a "retired" bounty hunter who's enjoying life in Jabba's palace, with fancy colours on his gauntlets / jetpack (which are, to me, totally useless for a bounty hunter to be that visible... At least the green gaunts / jetpack were kind of a camo).
Technically all the toys i had and the image i had of Boba was the ROTJ version, that's why i had a hard time choosing which one i would start. But the easiest way to decide myself was to look at Boba's scenes in ESB, then in ROTJ. No more hesitation, i would pick ESB and would be PROUD of it.
So here i am, starting my great journey with the Empire Strikes Back Boba Fett.