I started playing Dungeons and Dragons back in 2nd Edition, but it isn't my favorite rule system (I mean really, does anyone love Thac0?). 3rd Edition (3.5 or Pathfinder if you prefer) was the high point for me, in the dungeons and dragons franchise, even though it has its own problems. 4th edition just didn't interest me at all and although I've heard good things about 5th ed I just haven't gotten around to playing it at all.
While I like a lot of different systems for a lot of different reasons, my two preferred systems for fantasy games are Hackmaster 5th Edition, and Lamentations of the Flame Princess (LotFP).
Hackmaster 5th Edition. Still in its relative infancy, the core books are all out, it was born from the previous edition that spoofed a lot of the classic TSR rules and modules (and is actually a pretty good set all on its own), and their original western game Aces and Eights. What you get is a rule set that captures a lot of the feel of the original dungeons and dragons, but with lots of detail. The game repeatedly makes the point that it is a game of tough choices. You do have to choose between armor to protect yourself or the freedom of movement to not get hit. While it is fairly rules and dice heavy, there are a lot of tools to keep things moving and everyone engaged during combat. It provides a fine level of granularity to the game. The gamemaster book has great advice on how to set up adventures and make them engaging and award players not just for combat but for clever thinking and story advancement. The only downside of the game I would say is that I am too old. I don't just feel like whipping up a new monster or converting stuff over to the rules from other systems.
On the other end of the spectrum is LotFP. It is an OSR clone that takes a very rules light approach to the game. When I first introduced it to players some balked at it because the fighter class was the only class that advanced in combat abilities. I saw this as a feature though, as in many other systems the magic user classes soon eclipse the non-magic classes in versatility and power. In this way the fighter remains the king of combat. The balance come in little things like there are no monsters with armor better than plate mail, so even a non-combat class has a chance to affect physical combat at any level. Pretty much any adjustment I need to make on the fly during the game can be made with a +/- 1 or 2 to a roll and move on. The adventures published by LotFP is where the game really shines. Most of the adventures are system neutral and so can be easily ported to any game (I might even take the effort to port them to Hackmaster!) because they aren't filled with combat for combat's sake. If a monster is present, there is generally only one and it is an important part of the story rather than just an encounter. Like Call of Cthulhu though, the adventures are harsh and unforgiving.
The downside is that I have had players quit because of these adventures.
There are plenty of other systems that I like for specific games too. Usually because the rules provide incentive to, or not to do certain types of behavior in the game, or help establish the feel the game is going for.
I like Deadlands classic for this reason, it uses a poker decks and chips to assist in the game mechanics, which provide a unique system that helps with the whole wild west theme. It is far more complex then the streamlined savage worlds rules, but I think it also provides a lot more depth. One of my favorite bits is the use of "wind" and "wounds" which is much like non lethal vs. lethal damage, but also handles the wider range of terror, needing to catch your breath etc. compared to a broken leg or a shot up gut.