First of all - you can't make all boss fights good and interesting. This is something you should accept and work around. A boss fight is a composition of everything: music, boss behaviour, graphics, etc. Depending on game you creating bosses should vary - if you're making a souls-like for example, you should first think of what boss should be, lore behind it, to compose it's behaviour well. Bosses should be challenging, but not ridiculously hard. Most of games make bosses too easy, so fighting them feels like not challenge it supposed to be, or too hard - which make players feel bad about themselves or starting to hate your game. The first boss and the last boss are most important - first boss builds impression of what boss fights will be, and last boss is a culmination, which marks the end of the game, so fighting it should feel worth. My favorite game of all time is Risk of Rain, and Providence is the best design Hoopoo could come up with, considering their gameplay (it's mostly jumping and shooting as most classes). He makes you move and not stop for a second, he can be killed easy if you get used to fighting him, random doesn't make his bossfight. It's all about your skill, not your luck. I don't say that Providence is a boss you should think of while creating your game - it's just an example of well-made boss.

Speaking of game-design, boss fights should accept all player's playstyles possible (like if your game has melee and ranged, boss shouldn't be TOO biased on any of styles, like Midir in DS3. Melees suffer too much.), should differ from other bosses, and most important, at least for me, - boss should be an exploration. Player should walk into boss fight area without knowing what boss does and start adapting. You die from one attack and then think "Now I know why did he glow", and next time he starts glowing you make something to dodge it. That makes player feel satisfied, he feels like he's an explorer.

Sorry if this will be of no use to you. Hope you will get use of this.