Zardnaar
Legend
My Rank from Least to most complex:
Holmes is omitted as I've never used it (not because it's bad, but because it's only level 1-3).
Least Complex...
B/X - Only 2 thin books needed. It's called basic for a reason. The version I love the most - but I'm an OSR-player so it's not surprising.
RC - I don't have BECMI, but RC is almost the same thing. It has more levels than B/X so it's more complex.
ODD - A strange beast. I'd say it's unplayable without Chainmail, Greyhawk and Blackmoor - but with those it's not that complex (if you are able to understand the badly written text).
5E - With the 3 core books it's back to basics.
2E - With only the 3 core books it's 1E, but streamlined.
4E - I personally hate this version. The rules are quite simple, but it becomes quite complex when you add different powers together during a fight.
3E - It has rules for everything, and that's too much. And we are only discussing the core rules, then we add a bookshelf of splatbooks.
1E - The game I started with during 1980s and the reason I still love DnD. We tried to play the core rules RAW a couple of years ago, and it was horrible. Have you ever tried to play 1E RAW? Have you ever tried to use weapon speed? Have you ever tried to use AC adjustment? Have you ever tried to go back to attack matrix after you learned THAC0 and acending AC? Horrible!
...Most Complex
I used most of that in second Ed.
Weapon speed in 2E is optional, spellcasting times are not.
2E us a lot more playable raw. If I had to run 1E it would be like advanced B/X.
1d6 go and more races/classes. I don't think anyone ran it raw.