Hmmm tough question indeed for there are a lot of factors that I find rewarding in my collecting. It's hard to quantify/qualify which is on the topmost when it comes to "most rewarding" category.
I guess the most rewarding for me is the "high", adrenaline rush, excitement... call it what you will but that good feeling when you get a toy you wanted. That would be prolly the most rewarding for me.
Whether it's completing a series to a certain extent, getting a much sought after figure or completing a figure by finding that missing weapon or accessory, yep that high, excitement and good feeling you get IS the most rewarding ever. You can't top that, very positive emotion, for me.
Wanted AM partner Vanguard, Myclones Dirge, G1 Victory Leo, e-hobby Dark scream ( the black version), e-hobby Magnificus
Parts- AM partner Basher-side guns, G1 Actionmaster Elite Windmill's blades[I][B]