I find bosses really annoying, never played a game that it added anything or made the game more fun. It's just there to slow down the pace and interrupt the story. Sometimes I just change to easy difficulty so I can get over with it quickly and move on.

Boss heavy if the bosses are done right. Dark Souls 1 and Metroid Prime 1 are just a few personal favorite examples.

I hardly ever really liked any of the boss battles I played, so I'd prefer boss light games or even games without bosses. I do appreciate games with a huge variety of opponents and several tougher ones among them, but I don't really need those tired old scripted battles that work with specific attack patterns and weaknesses, that prevent you from using your usual abilities or tactics, that are partially like puzzles, requiring you to find out what the game expects of you, and that try your patience with lots of repetition if you fail to defeat the boss at your first try. I also hate bosses with lots of health and all kinds of immunities and damage resistances. It's just no fun when these battles go on for ages. Also, if every second fight or so is a boss battle the games become like a dull slog to me. If there are boss fights, they should be rare, so they will seem more epic and special when they occur.

I don't generally like games with bosses. I like it better if there is a nice linear advancement in enemy strength, not some individual spike.

I prefer bosses to be used sparingly, and I prefer them to not be frustratingly hard. And if the game doesn't have quicksaves, I prefer there to be savepoints immediately before and after the boss fight. If there are multiple stages to the fight, then at every stage.

I like games where boss fights are boss fights. They are memorable, they are incredible. Dark Souls has a boss in almost every section, yet every boss is a legit challenge. If the bosses were less frequent, then I wouldn't be bothered at all. If the boss fights are good, I don't care how many there are. If they're bad, I want fewer, even if there's only 1 to begin with. Quality over quantity.

depends on games, But half and half I would say is a good point. having too much bosses is more for an mmo or Jrpg style game and more for the adventure style having one once in a while is great.

Boss-heavy, but not on a simplistic schedule. None of this "oh, looks like the last room of the dungeon, time to file a quarterly report, er, fight a boss."

Depends heavily on the genre, but I do like a certain amount of unpredictability (like a couple people have already mentioned). Like reaching a place where a boss fight could easily be and instead grabbing whatever I'm after and getting away scot-free. Okami did that at one point and it was awesome. Or walking along a street that's been safe up to that point, only to have a giant robot drop down on me from the sky. Because if I were a giant robot, I certainly wouldn't be twiddling my thumbs in a large room somewhere while jerks ran around undoing my plans.

It's always nice when a developer includes boss fights because they make sense in the game/story and not begrudgingly as though they were filling out a checklist or padding out play times. Generally, though, I think I prefer fewer bosses because more time can be put into them and they avoid becoming fatiguing. There are enough games out there that have you fighting the same boss multiple times or palette swaps that it's become easy to appreciate games that don't bother making their lack of content your problem.

I don't like boss fghts in FPS games anymore. Today I dislike having them even in the original Doom games.
I still enjoy boss fights in certain 3rd-person action games, like Dead Rising and Earth Defence Force.

As a kid I always looked forward to boss fights though, it was exciting and in some ways the coolest part of the game.
Today, they are mostly just "speed-bumps" that put an awkward, unnatural and annoying pause in the game-pace.

Well-done boss fights are FUN and satisfying.
Simple bullet sponges foes are annoying, instead.

I like them if they're challenging, require some new strategy and possibly offer some reward.

If those bosses give indeed valuable loot, then I wouldn't mind meeting them in every dungeon. But usually I prefer fewer bosses, real hard bosses. Because I like exploration more than fighting. If possible, I would sneak by and steal loot. No time to deal with hoards of somewhat overpowered monsters.

In general I dislike boss fights, especially if they require different skills/tactics than the rest of the game (such as an attack routine where you need to wait to attack at a certain time and/or can only attack certain vulnerable locations). If bosses have to be present, I prefer them to be more like a tougher variation on standard enemies.

I would love a game with no bosses at all... it's a concept that doesn't need to exist anymore. With a good enough story you can end a game with so much emotion and power there is no need for a boss fight. Heavy Rain is a damn good example, Red Dead Redemption is another... You could argue the Metro games didn't have bosses either.. these all had amazing stories that carried it though...

Hell look at Oblivion or Skyrim... you by right would never need to fight a boss at all and still fill fulfilled after a few hundred hours of playing. The Assasins creed games had "bosses" sort of but they were more story driven fights that drove a point home, easy to beat and served a purpose to move the game forward.

Prince of Persia reboot was an auto kill boss and felt out of place... actually a lot of the end bosses did... That said the most pain in the ass fight I ever had was Ares in God of War 1... Screw that guy!

Might I suggest Ultima 6? Ultima 4 and 5 are also worth checking out. (Well, technically, Ultima 5 has 3 characters who could be considered bosses that you have to destroy, but you can't do so in normal combat (even if you do, the game will act like that creature is still alive); without spoiling anything here, let me just mention that the way you have to "fight" them is more like the way you fight enemies in pure adventure games.)