It took me a while make up my mind on Majora's Mask. I went from hating it when it came out to considering it the best of the entire series. Yep, above OoT, LttP, and BoTW. Yes, even tho Princess Z only has a short cameo in it.
If this review seems vague, its because I'm not trying to give a summary, or spoil it for those who haven't played.
The game's mechanics and execution are unique and unmatched in the 18 years since its release. The innovative idea of constant time travel back to the past in order to go to the future was ingenious. Termina looks small, with only 5 main locales (including Clock Town) but in reality it is massive. Part of the reason this game stands out amongst Zeldas and all games is its weapons' system.
Side quests (this game's most polarizing aspect): The "side quests" are not truly side quests. Even today, this is rare. Hence why the NPCs actually evoke feeling.
Dungeons/levels: One of the best parts of this game is how the pre-dungeon's tie into the dungeons, which are challenging, and most importantly, interesting. My only complaint is that there weren't more of them!
Bosses: Ditto above, could've used way more of them.
Music is easily the best in the series, especially when one considers the short time frame the devs had to get this game released following Ocarina. The original songs are still Zelda's best, as well as the parts of Ocarina it does reuse.
If you have a working N64, find a copy of the cartridge. Or if you have a 3DS, get the 3D version. The phrase "they don't make 'em like they used to" will have a whole lot more meaning.