Favorite Edition: That's a tough one. Basic is where I started but didn't ever own or DM it. My Thoughts: It was great for opening my eyes, and many others to RPGs, rules are light requiring many, many house rules. Good system to start with.
First Edition, or AD&D to the originals, I got my first few books and they were exciting. Lot's of material for spells, campaign worlds, deities, class ideas, new magic items, new monsters, and you can't forget the awesome random dungeon generator. My Thoughts: It was a vast and much needed upgrade to the previous system, probably the best upgrade to the entire system to date. Still suffers from many house rules needed. Great game to start with.
Second Edition shouldn't be called second. It honestly should of been more of 1.5 Edition, but let's roll with it. This is where my paychecks disappeared towards. I loved the three ring binder monster manuals. Insane box sets, great campaigns, massive dungeons, the the whole set of the Complete books (Some were crap but most of them had great material), Non-Combat skills (Most were crap but some were great and still a good idea), new races, new classes, new monsters, and the list goes on and on. My Thoughts: It was a nice and much needed update to AD&D. Still using plenty of house rules, but most game breakers are covered. Really great system to start with.
Third Edition and 3.5 were awesome on the start. New classes, Half-Orc is in, new skills, feats, saves are streamlined, great rules for building... well basically everything from magic items, to empire building, and campaign design. The big down-side was combat relied heavy on miniatures. OGL and D20 system changed the face of RPGs and whether you like it or not, it opened the door to the OSR movement and the massive amount of retro-clones we have today. My Thoughts: It was a major game changer to basically everything we had in the game community. The system had flaws but it was clean and quick from combat, to saves, skills, and character creation. They built a system that with some tweaks could turn into the only system of the future. Amazing system to start with, but if I want mini's I'll play Warhammer 50k.
4th edition and from everything I've heard about 5th, sorry D&D Next, basically seemed too video gaming to me. In my opinion is they had a great system for a basis with 3rd edition but went the wrong direction with the system.
So I guess my favorite system would be second edition, it was flawed but had ample info available, third edition would of been my favorite if the system had leaned a little less on game pieces.