Easily Mario Kart.
Mario Kart Wii - ~500 hours
Mario Kart GC - ~100 hours
Mario Kart 64 - ~100 hours
I have also spent small amounts of time on Super Mario Kart and MKDS, but I don't own them so less than an hour for each.
Zelda probably follows:
Twilight Princess - 180 hours
Windwaker - ~200 hours
Ocarina of Time - ~150 hours
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I generally don't buy that many games from the same series though, so it's hard for one series to excell over another.
I would guess Resident Evil I have played 180 for RE4:Wii, and maybe 100 for REmake.
F-Zero X probably 150 hours, plus 100 for F-Zero GX (and 2 hours on F-Zero from the SNES that I have on VC)
Banjo Tooie alone probably comes close to 200 hours, and Formula1 WGPII probably 250 hours on it's own too..... I doubt I have played over 200 hours on any other game series.
