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 Level Ever: Sewer Speedway. The best part is a half-pipe section including a wild shortcut that requires you to hop off the top of the halfpipe with a drift boost (or item) to reach it.
Most of the late-game tracks as well, notably Oxide Station the longest in the game. Especially once you know the Easy Level Trick for it.
Character Tiers: It varies from track to track. Some of the more straightforward tracks require a bit of top speed, while others are more favorable for karts with better acceleration and/or handling. Most Battle Mode tracks require good handling in order to be agile against most projectiles flung at you by other people, while most other kinds of tracks can have handling issues be circumvented a la power sliding to get proper boosts to not fall behind.
"Funny Aneurysm" Moment: According to the "Where Are They Now?" Epilogue, Pura gets a job as part of a Sigfried and Roy expy's act, which ends when he accidentally kills the Roy expy. Four years later, something similar (albeit not as fatal) happened to the actual Roy, ending the duo's stage career.
That One Level: Dragon Mines, which happens to coincidentally be a track which you have to challenge a boss in (Komodo Joe). While power sliding in a straight line helps mitigate some of the difficulty within the level, there are few straightaways and you also have to watch out for mine carts which get in the way at inconvenient times.
Hot Air Skyway, for the same reasons as Dragon Mines and more. Except for a few portions of the track, Hot Air Skyway for the most part has no walls. An inexperienced player will have difficulty with Pinstripe (the boss you confront in Hot Air Skyway) because they will have trouble with keeping themselves away from falling off the narrow roads. Pinstripe himself is not too difficult, but his bombs offer no favors due to how narrow the track itself is. There is a way to circumvent a bit of the difficulty via jumping across the other side of a track at a point where one of the walls is the only thing between two close portions of the road running parallel with each other.