D&D 4e.
It does fantasy heroics better than anything else on the market, the mechanics actually work (with the math fixes), and what issues it does have are much easier to rectify than the alternatives.
I get why some people don't like it, but I've never found my immersion to be particularly fragile so my group enjoy it just fine.
I don't know that I have a single favorite. There are things that I like about most of them I've tried.
D&D 5e is a really easy entry point into tabletop roleplaying, and I like that player characters get some new feature whenever they level up.
D&D 3.5e is a nostalgic favorite because it's what I got my start with, and as a DM I like the process for creating NPCs being basically the same as a player character, and Pathfinder is basically 3.5.5.
The Dragon Age RPG has a great core mechanic. 3d6, with one of the dice being a different color than the other two. If you get doubles on any two of the dice, you get get "stunt points" equal to the number on the off-color die, which you can spend for a variety of effects, including dealing extra damage, getting an extra attack, disarming your opponent, repositioning yourself or your target, etc. Unfortunately, the rest of the system is half-assed/tries to emulate DA:O too closely.
The A Song of Ice and Fire RPG recognized from the outset that, in addition to personal-scale combat, it needed to include rules for mass combat, fiefdom building, and "social combat." The problem is that each of those subsystems is too shallow and easily gamed.
I love GURPS 4E's modular depth -- you can play at whatever depth of rules complexity the group wants (I like pretty deep rules). However, creating monsters from scratch can be a pain, and for groups that want really tactical combat, mapping in hexes can be burdensome.
3.5 because I hate medieval fantasy
>Medieval fantasy
>With platemail and guns
Burning Wheel, followed by Torchbearer.
When I'm running 5e has become my favorite. It's easy to get people into tabletop games with it and it cuts down on a lot of the burdensome things like grappling rules.
I still enjoy playing in a pathfinder game that's been going for almost 5 years though. I think playing in 5e gets stale, even if you adapt a lot of older content to it.
I haven't even read the rulebook for the Song of Ice and Fire RPG. Is it worth cruising through even if it's just to pull ideas and mechanics to introduce to another system?
This. Simplicity is exactly what I look for in a medieval fantasy RPG. I leave the granularity to the grim dark Dark Heresy, and Star Wars for more interesting stories. Our players want medieval to emulate the "dumb mindset" of the era!
4e DnD.
Great for heroics and enjoyment for the entire group.
>The Dragon Age RPG has a great core mechanic. 3d6, with one of the dice being a different color than the other two. If you get doubles on any two of the dice, you get get "stunt points" equal to the number on the off-color die, which you can spend for a variety of effects, including dealing extra damage, getting an extra attack, disarming your opponent, repositioning yourself or your target, etc.
This is super cool and I'm going to find some way to incorporate this into GURPS.