I have to go with Pokemon on this one. Minecraft has only really been around for a few years (Really only got really popular a year or 2 ago and only officially released last year) so it's still fresh on everyone's minds. In Minecraft, you have the option to give yourself anything in the game with what you're given. Once you do that, you take away about 2/3 of the game. Making whatever you want is cool, but the novelty of it wears off and it becomes boring and playing survival mode isn't really all that addictive once you've tried creative mode. Multiplayer usually means finding a server which in my experience would have rules which made trying to make things with friends boring as you had to earn money to buy stuff to make things. Then there's the alternative of using Hamachi or another program like that, but that becomes boring as well when you only have 4 or 5 friends trying to make a bunch of big stuff and realize that you guys are most likely going to be the only ones who are going to see all the stuff you worked on. Mods breathe new life into the game for a while, but most aren't anything major. Most of what I've seen is stuff like added climbing controls, slide controls, fixed animations, etc. and the ones that are close *coughcoughPOKEMONMODcoughcough* aren't finished/multiplayer as far as I know.
Pokemon, on the other hand, has been around for almost 15 years now. Sure it's not as big of a deal as it used to be with the theatrical releases, religious protesters, etc. but it's card game and anime are still around so somebody has to be buying/watching it. The games are where it's at though. The games are still the reason a lot of people even buy Nintendo's handhelds. What makes Pokemon so addicting though is it's competitive aspect. The more friends you have that play Pokemon, the more competitive you get, and the more competitive you get the further in the hole you find yourself. It starts out casually enough, playing with the team you've made through the single player game and battling together, then training them to level 100, then making a better team using all the Pokemon at the game's disposal until one day you find yourself riding a bike back and forth in front of the daycare trying to hatch a Pokemon with the right nature and IVs so you can go out and EV train them so that he'll be just that 1 stat better than your buddy's. Even if you go out and buy a cheat device, it doesn't take away the competitive aspect of the games, just makes it easier and cuts down a substantial amount of the work. Even then, you can find yourself addicted to just making different teams to switch out and play with (I got to the point where I made a Sandstorm Team, a Hail Team, a Trick Room Team, and some others back when I was really into it). Minecraft doesn't really have that. Sure you can "compete" to make the coolest building or whatever but that's not really a win/lose scenario as it's more about opinions than anything else.
Of course, both of these aren't nearly as addictive as WoW (As much as I hate it), but this is what the debate was.
Edit: Just to add some information, I spent over 300 hours on Pokemon Diamond alone. I don't even think I spent 30 hours in Minecraft. Having endless possibilities doesn't always make a game more addicting than a game with a somewhat conceivable end point. Throwing someone into a game with no goal really and saying "Have fun" can be fun for a while, but eventually I find myself saying "There's really no point in this, if there's no end that means there's really no true progression so I've seen all there is to see" and put it down. Pokemon has goals that once you've completed you can go to the next one. Beat the elite 4, complete the Pokedex, get master ribbons, beat the battle towers, fight the online battle towers, make the best team you can think of, make the best team you can think of with a theme, and so on. That may not sound like anything on paper, but in practice it can take hundreds of hours. And that's just with one game. The great thing about Pokemon is it has so many games. It's feasible to buy Pokemon Sapphire/Ruby, do all of this in that game, trade them to the next game, repeat the process adding the Pokemon from that game to your party and repeating this until you get to the latest game, Have Pokemon with ribbons from all the games up until that point and perfect stats. Sure this may take you an ungodly amount of time, but I'm sure there's people that have done it over the course of the past 10 years since Ruby/Sapphire came out.