Hmmm.... the oddest boss would be slime boss from Secret of Mana which gets bigger and bigger as it takes more damage. So big that my team couldn't barely move at all with the whole room being taken by the slime boss.
When it comes to most memorable boss, there are bosses memorable because of bad things but there are ones of good things. My vote goes to Seven Force from Gunstar Heroes and Gunstar Super Heroes. Fighting boss with multiple forms each with its own attack pattern is memorable and challenging.