The desired hierarchy, initially hashed out in Proposed tag synonyms for [dnd4.0] but having evolved a little, should be and mostly is already:
- dungeons-and-dragons - For general cross-version stuff
- odnd - The first D&D, aka 0e, brown box/white box from 1974
- dnd-basic - The first Basic D&D by Holmes from 1977
- dnd-bx - Moldvay Basic and Cook Expert from 1981
- dnd-becmi - Mentzer Basic 'Red Box' + (expert, companion, master, immortal boxes, and the Rules Cyclopedia combo thereof) from 1983
- adnd - Advanced Dungeons & Dragons, aka 1e, from 1977
- adnd-2e - Advanced Dungeons & Dragons Second Edition from 1989
- dnd-3e - Dungeons & Dragons Third Edition from 2000
- dnd-3.5e - Dungeons & Dragons 3.5 Edition from 2003
- dnd-4e - Dungeons & Dragons Fourth Edition from 2008 (includes Essentials)
- dnd-5e - Dungeons & Dragons Fifth edition from 2014, also referred to as D&D Next in 2012. We have the dnd-next-playtest tag for questions that were/are specifically about its playtest stage.
- pathfinder - Fork of D&D 3.5e by Paizo in 2008
- And various retroclones.
dnd-basic was being used inconsistently and often redundantly with dnd-bx and no one was using the right Moldvay tag because it was bx-dnd instead of dnd-bx (I synonymed it). I propose that we either a) make dnd-basic just for Holmes or b) make it a collection of all three Basics, though I am unclear what purpose that would serve. or c) remove it in favor of the three specific tags.
In general you shouldn't overtag - dungeons-and-dragons for instance shouldn't coexist with more specific version tags, it's a halfway step between [system-agnostic] and a specific version and is for questions that cross more than 2 versions (if you just want to compare 3.0 and 3.5, then just tag with the two of those and call it good) or are general and not version specific, but are D&D domain specific.