If you had the chance to play any boss in-game for 24 hours, which would it be and what would you do?
During your period of control, the boss can be extracted from its instance/raid and utilised in any environment (e.g. world play or another instance/raid if so desired) including battlegrounds. Note that you would be an enemy to both factions.
As a means of discouraging everyone from simply selecting the Lich King, let's assume within this hypothetical that health and damage are roughly static from boss to boss and equate to a stats range typical of Ulduar 25. This query is more focused on your interest in boss abilities (and nostalgic value, if appropriate) than the damage/surviveability of one boss in relation to another.