Out of your list, I'd have to say Mario (at least the platformers) simply because they've stayed consistently similar but good throughout their life. However, I find the new 2D Mario games pretty boring these days so it wouldn't be my personal pick.
Since you mentioned consistency - which I'd agree is a factor in picking it - makes it so much harder. For example, so many series I used to love died with more recent releases - the original Spyro Trilogy are three of my favourite games ever, but the new stuff is trite. Some series just die out altogether - Wild ARMs.
So, of the games I play, it would come down to two - WipEout and Ratchet & Clank. Yeah yeah, I know, both small Sony franchises - sue me. I've played one bad game in both series (Fusion and All 4 One) while I've really loved all the other games equally. Certainly in terms of consistency they both deliver. (edit: LittleBigPlanet is well on the way to getting there too, but 2 main releases and a portable spin-off isn't exactly a big enough franchise just yet)
At the end of the PS1 era I'd have said Final Fantasy. Every single main-series title up until they had been absolutely outstanding quality. But I didn't enjoy X at all and haven't really liked any of them since then. Based on past merits though, it would have been FF.







