In addition to the ones warelander posted (which are very good), I have maaaaany favorites to post, so brace yourselves.
An unforgettable title song! It symbolizes everything that's fun and quirky, like the series WarioWare is.
And it inspired this amazing remix in Smooth Moves.
And it's part of Dr Crygor's theme, where it gets even more wacky.
Wario-Man remixes it in D.I.Y. Showcase! Wait...I can't find a video for it. =/
D.I.Y. also has a great title song!
Dribble & Spitz's greatest hit, in a special Nintendo album version. Wait...I can't find that version, so have the game version instead:
https://www.youtube.com/watch?v=poNlOFgSIf4
Really beautiful DriSpitz song from Smooth Moves.
https://www.youtube.com/watch?v=PsNp1Fx8rnU
And now the cabbies are present in a badass tune that fits their cool style.
https://www.youtube.com/watch?v=aM0HD4NOeYM
But they're so varied, that they get to have an epic space song as well.
Go Dribble! You're too cool to lose to girls! Wait...I can't find a video for the song of that Mona vs. Dribble mini-game from the first game. :(
https://www.youtube.com/watch?v=Ysx8ToBmydw
Japanese version of Body Rock, with lots of fun Engrish (sadly video leaves to be desired though, as it misses the first few notes).
https://www.youtube.com/watch?v=H2MO_4iCgEg
https://www.youtube.com/watch?v=DPfXMz2zPRI
This ended up being Jimmy's main theme. It's worth of being so.
https://www.youtube.com/watch?v=7XmBk9bGkCs
But this Jimmy one still rocks.
https://www.youtube.com/watch?v=0w0-Kyocm9M
I loved this multiplayer game in Mega Party Game$. The turtles are so cute!
https://www.youtube.com/watch?v=aJp2A80CXAY
I liked how this boss was so cute and fun, while Kat was so intensely focused in battling the skeleton.
And this one was hilarious. Wait...I can't find a video for the song that plays in Kat&Ana's Touched boss microgame...
https://www.youtube.com/watch?v=-QY_NOqIbe4
The only good version of Ashley's Song to me. Can't beat one of the most famous Japanese voice actresses.
https://www.youtube.com/watch?v=kJb0L_OYO0E
Last level of Game & Wario's "Ashley" had the best music to me.
https://www.youtube.com/watch?v=XNyFnM5XVSc
I love Mona's theme, because it's based on the Shokorakeeper's theme in WL4.
https://www.youtube.com/watch?v=HPh_tkmSFT8
https://www.youtube.com/watch?v=M4WaIfzpmlM
https://www.youtube.com/watch?v=Ey-4GbG8pYM
https://www.youtube.com/watch?v=YbnUpCzv1xk
Mona's take on some musical classics! I wonder how the European geniuses who composed them would feel about it? (I wish the videos had the random fun sounds you can make with the mixer...)
https://www.youtube.com/watch?v=66gRU4Otb8k
Orbulon's theme. So mysterious!
https://www.youtube.com/watch?v=fVbcZOY8aw4
And I just loved this one. It made that silly RPG battle even sillier.
https://www.youtube.com/watch?v=QkL5YfF978k
https://www.youtube.com/watch?v=39HOdGl177A
The first WarioWare and its GCN version had the best credits, in my opinion. With the latter being even better.
https://www.youtube.com/watch?v=S4FaJVzwoFw
Very neat remix of WarioWare Inc. Medley.
https://www.youtube.com/watch?v=2hKMErs_qXU
Listening to this while seeing the Dr. Wario title screen brings a smile to anyone's face.
https://www.youtube.com/watch?v=Nhg1YHb7x5I
Never complete without the main theme of the hero of this show.
https://www.youtube.com/watch?v=eiYU7loO4Kg
...But he has to share the spotlight now!
https://www.youtube.com/watch?v=5CYdPKssgbE
My favorite of the "all microgames" medleys.
Wario jumps the rope to a chill, WL4-inspired song. Not sure which level it got of from though. I think Wildflower Fields? Awesome regardless. Wait...I can't find a specific video for it, once more. :|
https://www.youtube.com/watch?v=OFaAfhEYLfU
https://www.youtube.com/watch?v=ATB9zK7rcTo
https://www.youtube.com/watch?v=P-2ZK6sPIRc
9-Volt's takes on good old SNES Star Fox.
https://www.youtube.com/watch?v=v2XgAkZ5NOE
https://www.youtube.com/watch?v=Gtytx3AKvVc
https://www.youtube.com/watch?v=GM0SQxGIAZY
https://www.youtube.com/watch?v=QAfYiR5KhJ8
9-Volt's take on Mario Paint, Super Mario Kart, Yoshi's Story, Wind Waker, four great games.
https://www.youtube.com/watch?v=LmdFV_q-0qM
And finally, 9-Volt respects Wario Land.
(also wish the videos had the funny sounds from the mixer, though I guess each one has his favorite sounds. I like Jimmy's "Oh yeah!" and that funny screaming one the most.)
I have more favorites, and I know I'm missing a lot of cool ones (especially from Twisted, which I played less), but this is what I can think for now.