Zelda imo, just because it's always quality and I'm not a platformer fan.
Mario Bros 3, Super Mario World, Yoshi's Island (does that count?) and Mario Galaxy were classics to me, but comparing it to Majora's Mask, Ocarina of Time, Link to the Past and Wind Waker there is a gulf in quality to me.
Mario is fantastic and always good fun but I don't find it offers an experience on the level of Zelda. None of either series are close to my favourite games but Majora's Mask would have to be damn close and I could give it for that alone.
Mario Sunshine was a bigger misstep than Skyward Sword imo (although both are good games). Same with Mario 2 vs Adventure of Link.