3.5 was my favorite. AD&D would be a close second. I bought the players handbook for 4. Never bought another thing of that version. The only reason I bought it was because I pre ordered it. That was a mistake. So unless version 5 goes back to 3.5 rules. I am going to be very skepticle. Pathfinder is basically the 3.5 rules. Pathfinder is a fantastic game. I do have a few complaints with it but they are minor enough not to even mention. lol.
T2K 2.2 and Traveller TNE are my all time favorite RPG games. TNE has FF&S rule book for designing your own equipment (guns, vehicles aircraft and spacecraft). What makes it even more awesome is T2k 2.2 and TNE rules are the same. So your equipment and characters can play in either game if you want.
