Krakatoa got all the media attention (I have the recent book published about it, but I haven't read it yet), but Tambora was the big one, at least in recorded history.
If you want to know about the really big one, check out what the Internet says about the Toba eruption, and then take a look at this map:
Lake Toba is the caldera from the Toba eruption. It's 1700 square km and 450 m deep. Now THAT's a caldera.