Absolutely Wario Land 4. I say this everytime a new member posts a thread like this, as WL4 will forever be my favourite Wario game.
It is definitely the best Wario Land game IMO.
Of the WarioWare games, I think Twisted is the best.
It's either WarioLand 4 or VirtualBoy WarioLand
I just love the underground ruins vibe so much and it really fits Wario being a treasure hunter.
Of course the levels are very well designed and makes me actively want to find everything. The games are short enough to enjoy over a weekend with tough content to keep me coming back.