The correct answer is Gal'Darah, the final boss that turns into a rhino. If you said Eck (and if the people I've done Gun'drak with are any indication, you DID) you are wrong, because Eck is as of 3.3 a horrible time vampire that steals emblems away from you. Same with the optional bosses in Old Kingdom, too. Oddly enough, people seem to be pretty keen on skipping the latter's optional bosses, but whenever I say "hey, let's skip Eck" everyone shouts LOL BADGES.
Let's put people in the LFG queue into two groups: those interested in Triumph emblems, and those only interested in Frost emblems. For the latter group, skipping Eck is a no-brainer, fair and easy enough. But what if you want that Triumph emblem? You still want to skip him, because it lets you queue faster to more rapidly get the two extra emblems for clearing a dungeon. The opportunity cost of that detour is greater than its rewards, simply put. Just kill the final boss, get 3 emblems, and queue up again. Eck is just not worth the detour compared to the three emblems that lie oh-so-temptingly close.
I would make a grand blanket statement about how killing ANY optional boss is a waste of time, but bosses like Infinite Corrupter and Elder Nadox are a little different. While optional, these bosses come with no trash, making blowing through them a relative breeze, and make good sense to do, if just for the cash value of 1/15th a crusader orb.
The one exception I can see is if you only have time for one, and ONLY one dungeon. Let's say you had a 30 minute window to play WoW; then killing optional bosses is fine and good, since that time doesn't delay any future dungeon or other profitable activity. I seriously doubt many people are in this situation, both for practicality reasons (if the dungeon gets stalled you're SOL) and more psychological one (you're unlikely to have fun while you worry about the clock).