These are what we call the 'YMMV items.' Things that some people find in this work. We call them 'your mileage might vary' because not everyone sees these things in the same way. This starts discussions in the trope lists, a thing we don't want. Please use the discussion page if you'd like to discuss any of these items.
Best Boss Ever: The bosses of the final stages usually fits. The first game had you play through a hilariously crappy adventure style game that switched controls on the fly to things you may have played through earlier. Touched also had a similiar set-up for Wario-Man's boss stage and plays Final Exam Boss a bit more straight. Tiny Wario's stage had Wario Dance Company where you had to dance to the beat and strike poses every now and then. From the first game, we also Orbulon's boss game in the first WarioWare where you have to defeat a boss Dragon Quest style by picking the correct spelling of the words, and Dribble & Spitz' boss stage which is reminiscent of the Shmups genre. Twisted had 9-Volt's boss stage where you played through a level from Super Mario Bros.Smooth Moves had Dribble & Spitz' boss stage where your Mii went through a training course stage before taking down a giant nose, and 9-Volt's boss stage where you played through a level from the SNES Star Fox (Corneria, Sector X and Titania on the respective difficulties) and had to take down ROB with a Zapper.
Best Level Ever: 9-Volt's stages tend to be much appreciated by fans for its focus on Nintendo games. From the first game, we have Ana & Kat's stage which is very Japanese flavored, having a song instead of interlude + game music, and having interesting microgames based on Nature. The first game also has Dribble & Spitz's stage for its Sci-Fi style and song rather than interlude + game music, similiar to Ana & Kat's stage. It also has a great boss stage. Smooth Moves feature Dribble & Spitz for having interesting games as well as the microgames' music once again being switched in favor for a song and having a great boss on top of that.
Broken Base: Game & Wario has rather sharply split opinions among fans and critics. It's either yet another fun if inevitably uneven experience or the series' Jumping the Shark moment.
Ensemble Darkhorse: For only appearing in a few of the games, Ashley is very popular among fans.
Game-wise, 9-volt and 18-volt are generally the most popular, as their microgames are based on classic Nintendo games.
Young Cricket is popular among the female fans for being the series' only Bishōnen.
Hilarious in Hindsight: The game which basically inspired Wario to found his company is Pyoro, a game for handhelds that's highly successful and features a red bird as its mascot. Sound familiar?
The description for the Kid Icarus record in Twisted is "This is as close as you'll ever get to a sequel." Nope.
Dr. Horrible's Sing-Along Blog has a character named Penny, who sings, has red hair, and a mad scientist is in love with her. WarioWare has a character named Penny who is a redhead, a mad scientist herself, and harbors a secret fondness for singing.
More like Dribble & Spitz's boss stage from Wario Ware Twisted. You must violently shake the system in order to outrun a boulder, while jumping over obstacles and holes. If you can SEE anything on the screen, you're not shaking it fast enough to outrun the boulder. Completely unplayable on level 2 or higher.
The slider puzzle in Orbulon's DIY microgames.
Ashley's boss game in Touched, in which you have to navigate Bullet Hell without getting hit once.