If I were going to start a new campaign I would almost certainly go with the Adventurer Conqueror King System for a ruleset. For me this game hits closer to the sweet spot between crunch, flexibility, and user friendliness than perhaps any of the other retro D&D-inspired systems out there. I was aware of ACKS when I started my current campaign, but decided not to adopt it because the core rulebook is still a bit too long and fluffy for my tastes. It contains a lot of material that is of extremely high quality, but is largely extraneous for my own campaign.
Damn. If I had only known about THIS:
The ACKS Character Codex. This book is a beautiful thing. It is the core of ACKS system condensed into a small digest-sized booklet. It's essentially a Men & Magic or Player's Handbook for ACKS. And it's only $4.45 on Lulu! (This isn't an advertisement, by the way - I have no connection to the creators of this item.) This means if a referee wants to adopt the system, they could afford to buy a stack of copies to hand out to players. The referee could then use the more voluminous core book as a reference.
This book looks great. Here you can see it compared to the Grey Co. Spellbook supplement and the original Men & Magic:
Here's the title page:
Tavis Allison tipped me off on this thing. Apparently the Codex is not considered an official Autarch publication, however Tavis tells me that an official version of the Codex is on Autarch's lengthy to-do list. Let Autarch know if you want more stuff like this. I do!