I'm currently re-reading my PDF's of the OSR games Dark Dungeons and Full Metal Plate Mail. The former is a clone of BECMI/Rules Cyclopedia D&D and the latter is a very faithful albeit generic clone of Original Dungeons & Dragons, specifically the original 1974 White Box set. I know Swords & Wizardry is the more popular and supported OD&D clone, but Full Metal Plate Mail is free while Swords & Wizardry is not, at least as far as I know.
While 3.5/Pathfinder and 5e have more polished and complete mechanics and are more current, there is a certain charm to the older TSR editions of D&D, as well as their OSR retro-clones. I can't quite put my finger on what exactly gives old-school D&D its charm, but I know it when I see it.
OD&D in particular is interesting because of all the early weirdness it had, being the first true RPG out there. It was very sandbox in nature, and the rules were so vague that it's easy to make homebrew material for it.
I also like the theory that OD&D was meant to be a multi-verse styled game that used medieval fantasy as the springboard. This is supported by the references to sci-fi elements in the original boxed set such as robots, androids, Martians, and creatures from the John Carter books.
Hell, I'd like to do a sandbox OD&D game (most likely using Full Metal Plate Mail) and play with all that 1970's pulp fantasy weirdness that was present in the original set. I might even include multiverse elements as the campaign progresses.
Ideally, I'd start an OD&D campaign with just the White Box materials. Three classes (Fighter, Magic-User, Cleric) and four races (Human, Dwarf, Elf, and Halfling) and then as the campaign expands and continues, I'll start adding in supplementary material and some homebrew works as well.
Everyone would start at Level 1 and progress from there. I'd try to do this as an online game and I might invite my fellow Kiwis as well, once I find a place to host the campaign.