Agreed with the above, not a great investment in absolute $$$ terms most of the time. If you definitely plan on doing it though, and i could be wrong, but i would stay away from TV shows mostly. Go for things that have appeal that spans decades. For me, since i only collect Marvel, i would say that, and it seems even boxes from the 90s with big print runs have held or risen in value. DC is probably similar.
Edit: damn i didn't know they pulped the MM95, whats the rationale behind this? Not devaluing future sets in the eyes of buyers?
Edit 2: Maybe big cult movie franchises like Harry Potter and LOTR might also be they way to go.