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.
YMMV: Sonic 3D: Flickies' Island
Best Level Ever: The Special Stages from the Sega Saturn version are a fan favorite, being a vastly improved version of the Special Stages from Sonic 2. These stages are rendered in true 3D and have more elaborate features like springs, gaps, and upper/lower paths. On top of that, they are arguably less headache inducing than the Sonic 2 Special Stages.
Some elements, such as the upper/lower paths and the animation sequences were retained in Sonic the Hedgehog 4Episode II 's Special Stages.
Broken Base: Suffers the same arguments as Sonic CD over which soundtrack is the better one, the Genesis or Saturn version.
Ear Worm: Despite not being one of the most popular Sonic games, it has one of the best soundtracks. Gene Gadget Zone in particular has music that's right up there with Ice Cap and Lava Reef as some of the best Sonic music ever.
Goddamned Bats: The snowman badniks in Diamond Dust Zone, which due to the isometric nature of the game are far more annoying and difficult to avoid/kill than any of the badniks in Metropolis Zone. And in this game, you have to kill them in order to complete the level.
Good Bad Bug: In addition to the Classic Cheat Code activated level select, there's a second way to trigger it. As part of a kind of safety feature, if the game's brains get sufficiently mixed up that it tries to execute out of range code, it'll automatically skip to "CONGRATULATIONS. YOU HAVE FOUND THE SECRET LEVEL SELECT SCREEN!". The Genesis being fragile as it is, the easiest way to do this is to simply smack the console (especially during the prerendered 3D animation at the start of the game). It was either this, or simply crash.
You can also trigger it in emulators by opening a savestate of any other game while you have 3D Blast/Flickies Island open.
Maybe that level was a little annoying but it's not really that hard, and the killer music made up for it. Volcano Valley on the other hand is a very frustarting Lethal Lava Land with a ton of treacherous jumps over lava, chances are you'll end up getting hit and losing your Flickies more often then any other level in the game.
Although if you can snag a Fire Shield and hang onto it, the level is a piece of cake.
Gene Gadget Zone Act 2 is also a pain because of one section where you have to climb up a large incline by jumping between moving platforms and dodging enemies. This section is designed to loop around until you can manage to get all the Flickies, and chances are it'll take more than a few tries.
That One Boss: Most of the late game bosses can be described as this, but Volcano Valley's is the worst. Having to cross thin pipes in an isometric perspective with a rather slippery control over your character with lava and a dragon following you makes this boss a life stealer. This can be dealt with by attacking him quickly, but usually youll end up getting hit and/or dying.