The side-scrollers of my earliest gaming days usually consisted of level-boss-level-boss-level-boss and so on. Many games still swear to this set up, just like a work of music ; verse-chorus-verse-chorus-(bridge)-chorus. I've always enjoyed a good boss fight. Super Mario Bros. had a fight after every level (not stage), which usually consisted of a battle with Bowser on a bridge. Still, you couldn't hurt Bowser, not even with the fire flower equipped, and had to jump over him and send him to his firey demise.

Some games have amazing boss fights, and later I will give you three very memorible boss battles, but first I want to talk a little bit about the bosses and their usage.

Bosses were used in the earlier games, the linear games, to end a level. After you'd fought an army of soldiers, you would end up fighting the captain, major, or general, in a sense. In essence, he was the lord, the boss, of that level. Bosses are portraied differently from game to game, but they are, for the most time, more powerful and have more tricks up their sleeves than your average thug. I will use many NES-games to justify these bosses.
The game "Trojan" is a game I played alot when younger, and it is a side-scroller beat'em up-like game that follows my instruction. First, you battle a horde of thugs, then you meet the first boss,a semi-boss actually, which is a couple of axe-throwing dudes with an own health bar, and a long(er) distance attack. This is how the game goes til the very end: Levela-semiboss-levelb-boss.
So I admit, is it just out of my formula.

RPG-games feature different boss fights, and the fights themselves often utilize a completely different approach each time, due to the notion of the rpg-rules. I haven't played many RPG-games, but I've played some of the earlier Final Fantasy-games, Chrono Trigger, Secret of Mana (both great rpg adventures) and some other odd games.

Nonetheless, thereare bosses in most games, but remember I say most games, NOT all games. Boss is a fitting name as in the earlier games, they actually sometimes were the bosses of the minions (Dungeon Keeper - Lord of the Land, Megaman bosses etc.). Now I will present three great boss fights I have encounter while playing games.

3. Metal Gear Solid - Psycho Mantis (PSone)

Metal Gear Solid is a fantastic game which really have a emotional and immersive story. Atleast this is what I think. A simple espionage action-game I feel all boss fights in this game are pretty awesome (Revolver Ocelot, Sniper Wolf). Pscyho Mantis was the third boss, I think, and is one of the FOXHOUND squad. He has psychic and telepathic abilities. The cool thing about this fight, is that, on the PlayStation-version he can read the data of your memory card and can tell you some of the games you've played. He also reads your data report so far, and how you have dealt with the game so far (based on statistics), and he makes the controller shake! This is pretty awesome! While I fought him, my controller suddenly got unresponsive, and I couldn't do anything - this was the work of Psycho Mantis... he set my controller port 2 as the active port, so I had to switch. This was a great fight, and a truly uniqe boss battle.

2. Half-Life - Tentacle beast (Pc / Dreamcast)

The bosses of Half-Life seem to be original and something that diverts from the original norm. Like in Super Mario Bros. you have to outsmart the bosses, and you can't just fight them with your regular arsenal. You encounter these/this three-headed tentacle beast... hmm, it's been awhile since I played Half-Life... anyway, you meet them in the game, and you can hear them banging against the walls. They are placed in some kind of test-chamber, and have entangled the entire room. The room itself is built like a giant sylinder, with ladders and walkways hugging the walls. Your object is to enter the sylinder-like room, scare the monsters, and fight two switches that are "gas" and "electricity" (I think). After finding these, and flipping them, after a long adventure into the depths of the lab, you can get back and incinerate the beast. This is a smart way of dealing with a boss, I'd say ; some games have you spending all your ammo on bosses, while other games let's your character act smartly and defeat the boss using the environment. Half-Life is still one of my favourite games. Wish they'd bring it back, in HD to the PSN and Xbox Live Store.

1. Shadows Of The Colossuss - All (PS2 / PS3)

I know, this is cheating (a bit), but, ok, If I had to choose, I would choose the Knight Colossuss, or the flying bird-like Colossuss. These "boss battles" I enjoyed for the share massiveness of 'em. Climbing the colossi and fighting them David vs. Goliath-like is great fun, and ... just great! The rush when sitting atop the head of a colossuss and he violently shakes to drop you off ; pure magic! This game is a game everybody should try and complete.