Which edition is better to buy Anniversary Edition, Special Edition or Legendary Edition? (The Elder Scrolls 5: Skyrim)

Whichever you like more, then it is better to buy

Better download, Todd Howard won't get poorer.

MayheMxyzeBurzum
If you are not interested in mods, then AE is better, because there is more content (quests, new armor, weapons) and new gameplay elements, if you are interested in playing with mods, then LE, you can of course also SE, but some mods will not be available.

Download Special Edition and don't worry. The remaining editions of the game are with installed mods from other players approved by Todd Howard.