What makes a boss good like in game? Like making it enjoyable or unique to a game?Cos im now making a 2D platformer game with a few bosses but i really need advice on a good boss design and why its considered good.Do leave suggestions on games i can use for references.Much appreciated.

This video by Skip the Tutorial might help.

Personally I like: 1. multiple boss phases, with increasing difficulty 2. unique way to defeat it   3. some element of surprise i.e. 'I didn't know it was going to do THAT!'

Designing bosses is really hard, we've just done that for our newest game and I've learned a few things:

• Old vs New Mechanics
  A bossfight is supposed to test your mastery of the games mechanics, but still surprise the player. It is important that you don't introduce complex or completely different mechanics in a boss fight, and focus on how you can enhance your mechanics that the game is focusing on in that stage. Having a platformer and introducing a weapon you can pick up in a boss fight feels like a disconnect, even more so if it includes controls you haven't used yet ("How do I pick it up? How do I use it?"). You can solve that by making sure control schemes are intuitive and reusable (my dash move gets replaced by shooting, and the weapon is picked up automatically). Don't expect the players to learn anything while in a boss fight. Teach them. Try to design your general boss mechanic around one extension of your main mechanic.
• Clear Feedback
  Can't your boss be hurt by normal means? What actually does damage the boss? Do I just need to endure this? Will I have to lure the boss into a hole?
  It is really hard to clearly communicate the means of defeating a boss, and this is probably the first thing to go wrong when you are designing a boss. If you do have a general effect of an enemy getting hurt, making sure the player notices if it is suddenly missing if they can't damage the boss by normal means. Knock them back, make sure they understand what DOES NOT work. As soon as they can cross off strategies to beating a boss, there is bound to only remain one. Make sure that the winning strategy is made clear early on. Level design will impact this a lot aswell. If your boss needs to charge into some hazard, make sure it is a hazard the player will identify as such, and really contrast them to the rest of the level.
• Narrative Design
  A part of what makes bosses unique is the build-up to the fight, maybe they are taunting you in earlier encounters or are set up in lore. A boss coming out of nowhere has no impact, even surprise bosses have been introduced to the player in some kind. Make sure the player really wants to defeat the boss for what they are, and not just because they are "just another enemy, but stronger". Having a rough understanding or narrative structures in storytelling might even help you flesh out multiple boss phases. New boss phases need to have a purpose. Did the boss lose a crucial weapon and is starting their desparation move? Awesome, the tension is rising! Did they just continue fighting, but now this phase lacks some attacks? Why even bother? Subverting expectations (once established) keeps the player on edge, and makes the boss memorable.

There are a lot of great videos on that topic, one I recommend is The Anatomy of a Bossfight by Adam Millard. It also has a lot of examles from Furi, which is a good starting point, since it's a boss focused game anyway. That does shift the context of boss design by a bit though, since permanent boss fights are not as climactic as boss fights interspersed into normal gameplay, another important bit of narrative design.

Best of luck! :)