Easily Firkraag from Baldur's Gate 2. By the time you fight him you're generally NOT prepared at all to kill him, and the game doesn't even really encourage you to - the option to walk away and return at any time is always there - but killing him nets you the absolute best weapon in the game, the Holy mother-effing Avenger.
If you're a Paladin then you absolutely need that sword. Seeing as my characters are always Paladins, I constantly find myself in the dungeon, hideously underlevelled and pooping bricks, loading attempt after failed attempt. Only through some cheeky use of traps from Yoshimo, my most furious kamikaze spells, the highest level summons I have, and Minsc wielding the mighty Dragonslayer axe can I take him down at that level. Thing is, I've done it countless times by now and it NEVER gets boring. The rush of seeing Carsomyr on his corpse and finally getting it in my sweaty fictional gauntlets never ceases to inspire.
Edit: I forgot two absolute corkers. First, of course, Dark Souls. Ornstein and Smough. Mother of God these guys are horrible. Fighting one boss in Dark Souls is pant-shittingly terrifying enough, but these guys? Forget it. One is a gigantic hammer-wielding ball of lard and anger, and the other is a nippy dragoon with the ability to blow you to pieces with a lightning bolt. Even if you manage to kill one of them - which, trust me, is hard enough - the OTHER one gets all his health back and gains the powers of his fallen ally. Giant sumo dude was tough enough to begin with, and then he gets imbued with the power of thunder and really starts to wreck your shit.
Next, I've got to go with something from WoW. The feeling of grinding your way through a new raid with your guildmates was fantastic, and that feeling when you dunk a boss that very little is known about, your victory scored in blood? Amazing. Victory is even better when shared through a battlecry over Ventrilo.
If you're a Paladin then you absolutely need that sword. Seeing as my characters are always Paladins, I constantly find myself in the dungeon, hideously underlevelled and pooping bricks, loading attempt after failed attempt. Only through some cheeky use of traps from Yoshimo, my most furious kamikaze spells, the highest level summons I have, and Minsc wielding the mighty Dragonslayer axe can I take him down at that level. Thing is, I've done it countless times by now and it NEVER gets boring. The rush of seeing Carsomyr on his corpse and finally getting it in my sweaty fictional gauntlets never ceases to inspire.
Edit: I forgot two absolute corkers. First, of course, Dark Souls. Ornstein and Smough. Mother of God these guys are horrible. Fighting one boss in Dark Souls is pant-shittingly terrifying enough, but these guys? Forget it. One is a gigantic hammer-wielding ball of lard and anger, and the other is a nippy dragoon with the ability to blow you to pieces with a lightning bolt. Even if you manage to kill one of them - which, trust me, is hard enough - the OTHER one gets all his health back and gains the powers of his fallen ally. Giant sumo dude was tough enough to begin with, and then he gets imbued with the power of thunder and really starts to wreck your shit.
Next, I've got to go with something from WoW. The feeling of grinding your way through a new raid with your guildmates was fantastic, and that feeling when you dunk a boss that very little is known about, your victory scored in blood? Amazing. Victory is even better when shared through a battlecry over Ventrilo.