this is really an impossible question. i got tired of phelps after the last olympics just from all the tv coverage constantly saying he is the greatest olympian ever. granted he's certainly up there. but it is so much easier to get multiple swimming medals than multiple medals in any other sport. not saying its easy, but it is actually possible. especially since there is much less recovery needed from swimming events.
phelps dominated his swimming events like no other. (well, i don't think he does anymore, but at least he used to) and bolt dominates his track events like no other. thats all you can really say. i think Bolt is more inspirational though cuz he's just destroying records. while records in swimming are pretty common because the swimsuit technology keeps getting better.