I will not say my favorite because that could change but I really like the Exalted Falchion. The Falchion and Parrallel Falchion are awesome as well (the Flachion turns into the Exalted Falchion and the Parrellel Falchion is the same thing as the Falchion.)
In my opinion, I would say that my favorite weapon would be the armadyl godsword from runescape 2007. I love it so much, that I got a custom sword made to look like the armadyl godsword.
This is a difficult question to answer with only one weapon. I've used so many that I consider formidable in games. Really can only break it down to M1897 Trench Gun and MG42 in COD: World at War, concussion rifle in Halo Reach, M4 in FarCry, M1 Garand in Medal of Honor/COD, and the shotgun in DOOM.
Of all the games I've played I liked most of the weapons in the Crysis games, the Stinger from Call of Duty, all the guns from Unreal Tournament, the hidden blade from AC for its style. And there are so many other generic types of which I can't recall any specifically good ones.
In a game called Vindictus I have always loved the battle scythes. They are really cool, and although Vindictus got bland to me after some time, I still think it has some awesome weapons and gear.
Honestly, looking back at some pictures really makes me want to try playing it again but it's a bit grindy for me.
My favorites nostalgically are the Master Sword from the Zelda series; and the first Gunblade from the Final Fantasy series, I can't remember the name.
I'm gonna have to go with any rocket launcher in any FPS, just for the rage factor.