For me, it's kinda tied with the Mario RPGs and Zelda series right now, with my choice of favourite depending on what I've been playing the most recently.
I prefer the Sonic and Mario series more. Though I prefer it over Kirby and it's definitely my favourite Mario spin-off.
The thing that I love about the Wario Land series is how innovative it can be. WarioWare though? The first was but the other entries feel a bit too similar layout wise, especially from Touched and onwards.