ooh boy. well, I'd say Wario Land 2 is probably the best one to play for anyone new to the Wario Land series, as it's the least complicated game and progress is not denied if you miss something. the first game is much like the second, but it feels more difficult to me. I mean, eventually in the second, you do have to focus on the large treasure map/getting the treasures, but it's also something fun to play that really won't put pressure on you first playthrough.
and any WarioWare game, but I find that starting with the first that was on the GBA/ported to GameCube is a good place to start. the WarioWare series sorta turned... "gimmicky" after the first game? Touched! is really great, though. Twisted and Smooth Moves, I love, but don't think they're for everyone. Twisted in particular. it had a motion sensor thing. you can emulate it, but it's... weird and not super fun, since the turning isn't going to be as smooth or precise.
and any WarioWare game, but I find that starting with the first that was on the GBA/ported to GameCube is a good place to start. the WarioWare series sorta turned... "gimmicky" after the first game? Touched! is really great, though. Twisted and Smooth Moves, I love, but don't think they're for everyone. Twisted in particular. it had a motion sensor thing. you can emulate it, but it's... weird and not super fun, since the turning isn't going to be as smooth or precise.