Actually, the first Wario Land is still very special to me. Back then it felt like adventure, Wario had his Greek fishing hat or whatever you want to call it, loved the different hats, great varied music... the only downside was the very floaty controls and problems with doing high jumps. That and I hated the boss for Parsley Woods. But I played that game on Super Game Boy, man.
Then Wario Land 2 came along. While I consider that game much better, it felt less adventurous (you never see a world map) and the music seemed more like derivatives of one theme. I can't remember (or distinguish from memory) half of the songs in it, they all seem similar. The game also took a sharp slant towards puzzles, which it had to because Wario could no longer die. Whenever I tried getting treasures, I'd get at least 200 coins so I could always put the speed on Slow to ensure I grabbed it.
Wario Land 3 was more of the same with prettier graphics and having to earn your abilities (which was a convenient way to keep you in suspense because you see something you can't quite get to yet -- it's the old Metroid formula again). Marginally better than 2. But darn it, I HATED that golf minigame, especially since it blocked your path to treasure chests until you nailed it.
Wario Land 4... eh, it was good, music occasionally rocked (
Crescent Moon Village is the best Wario Land song EVER, I was so overjoyed to hear it as the "High Scores" music (segment at 1:54) at the end of a Easy/Hard/Tough factory challenge in WarioWare Microgames), but I didn't like it as much as the previous entries. Granted it was up against what's essentially the perfection of the action-puzzle genre.
WarioWare is amazing all-around. Fun microgames, great music... but, ah, it's not a Wario Land title.