Opening boss fights make for very tense early fights in games that introduce you to the gameplay mechanics. My favourite is Scylla from God of War: Ghost of Sparta because of her size and the atmosphere of the fight and she makes for an amazing opening boss.

I think the fight against Poseidon on Gaia remains my favourite. It really kicked you right into the game and said oh yeah we are going all the way in this god of war game.

Vile,from mega man x.The fact that is impossible to win makes you feel weak,and zero saving you at the end also serves as an objective for you: become as strong as he is.Its by far the best use of a first boss to me.

It wasn't opening, but the first Titan in Unreal was so well done, first you see the Dark Arena from distance, then get to travel to it, enter and explore it, reading cool Nali messages about some frightening beast, and finally you get to the arena itself and walking to its center with mysterious construction and dead bodies - atmosphere that you could cut with a knife, adrenaline so high, you knew shit was going to hit the fan any second now, and when you finally overcame the fear and turned the switch, the ground started trembling and frightening sounds coming, the doors opened and in one of them the titan appeared, and you were so frightened, almost immobile from fear.

On my first playthrough I missed the prison cells under the Arena. Also I missed the Harobed village, and later turned out that the village had a semi secret graveyard behind it, and secret cave system under it. The game was like epic adventure, traveling alien planet from one place to another.

Bayonetta 2 for sure. I'm not sure if this counts as the first boss fight but this was my favorite.

the opening boss fight in resi 4, that lake monster that looked like a salamander.

Poseidon from God Of War 3 is my favorite, followed by the Asylum Demon from Dark Souls.

Yes, Poseidon on GOW 3 was mind-blowing at the time. It still is, actually. That was one hell of a way to start the game.

And to think that I find bosses in God of War to be pretty underwhelming in general... everyone here loves them xD

Anyway... I guess War from Darksiders II. It's a good old humanoid boss, offers a decent challenge for a first boss... and it's a pretty emotional battle. It has it all...

I like Darkside (redundancy ftw) from Kingdom Hearts too. More than the fight itself... the tutorial as a whole is excellent. The boss in the end is not that great, but combined with everything else... it makes for a great battle.

I forgot about that one. Gomorrah is an insane opening boss...