The Metal Gear franchise as a whole is almost unmatched when it comes to consistently well designed boss fights. MGS1's FOXHOUND bosses each had their own design philosophy, and their personalities match how each battle needs to be confronted. The most broken boss fight in the game is Sniper Wolf round 2, as the 1v1 sniping battle can be bypassed completely by using nikita missiles.
MGS2 bosses are much tougher than MGS1 bosses on higher difficulties. Olga on European Extreme requires excellent accuracy and excellent timing. Not only do you need to follow through with a strategy, your performance must be in top form. The Olga fight, harrier battle, and RAY barrage will push you to your limits and succeeding is incredibly satisfying.
MGS3 is incredibly varied when it comes to what strategies can be used. Each boss fight requires a completely different thought process, bum rushing The End won't work, The Fear requires fast reflexes, The Fury calls for hit and run tactics. The amount of tools you have and the varied conditions for each fight make for really fun battles.
Metal Gear Rising's boss fights are about fast reflexes and dominating your opponent with pattern recognition. It's all about speed and grace, planning a perfect parry ahead of time or bashing your opponent and risking a quick parry when the time comes.
