The D&D Rules Cyclopedia is, in my opinion, the best-ever edition of D&D, for a few main reasons.
First, because it is a single book that contains all the rules you'll ever need to play. It's a Players' Handbook, Dungeon Master's Guide, and Monster Manual all in one.
Second, because it was a consolidation of the Basic, Expert, Companion, and Master rulebooks (which, along with the Immortals expansion, are referred to as the "BECMI" edition of D&D). This edition of D&D focused on being easy to learn and play (hence being released in five different sets, allowing a group to add more advanced rules only once they're ready), putting simplicity first while still allowing for lots of character customization with the skills and weapon masteries you could gain as you leveled up.
And third, the BECMI edition had a very strong focus and core identity. Namely, it was a game about dungeon crawling (and hex-crawling) first and foremost, with rules designed to support that style of play. While 5th edition tried to make dungeon crawling possible, it's really not designed for that playstyle at all (while also having slow and clunky combat rules that make tactical fights less fun to play than in a game like 4e).
Add in the many helpful bits like the morale and NPC reaction rules (providing solid mechanics for roleplaying NPCs), rules for retainers (giving players the ability to spend their hard-earned gold on recruiting NPCs to accompany them on adventures), and rules for building strongholds and attracting followers (giving PCs another excellent "gold sink" and the ability to expand the scope of the campaign to domain-level play if desired), and this version of D&D succeeds in creating the specific flavour of fantasy it aims for to a degree never reached by any other edition (except maybe 4th, though that game aims for a completely different genre).
While switching to this edition of the game can be a bit of a culture shock if coming from 5e, I highly recommend it to anyone who wants a more streamlined game that has a strong mechanical focus.
