Of the series I play, Pokemon. I've played all of those series except Metroid, so I won't consider it to vote.
Mario is my favorite series, Zelda is probably second, and Smash Bros is too fun to get rid of. Pokemon used to be fun for me, but now I just find it tedious and boring. I don't even bother playing anymore and won't be sad if they stop making games.
As much as I hate to say it... Metroid. The first four are the franchises the casual fanbase are familiar with the most and any of them being lost would deal a lot of damage to Nintendo. Metroid however is not that popular to the casuals and losing it would still deal some damage, just not to the extent of the first four.
3DS Friend Code: 3179-6117-3933 PSN: ansonip2000 Official Take-Mikazuchi of the Shin Megami Tensei IV board.