A person who, by means of purpsose or not, uses sarcasm to have a laugh. A person who, on a regular bases, annoys the hell out of people with their sarcastic-attitude.

Someone who enjoys giving flippant, sarcastic and/or rude answers to legitimate questions; also someone who enjoys being sarcastic and rude in general. See also: wise-ass, smart-aleck, etc.

A mule/donkey of unusual intelligence

a person who persistanly uses sarcasm as a means of communication.

A person who just has to make everything make sense. Speaking of which, smartass is a oxymoron because an ass is a stupid person so smartass means "smart stupid person".
(not exactly the opposite of dumbass because a dumbass really is an idiot but a smartass isnt neccessarily a genius)

An annoying asshole who attempts to use sarcasm every chance that he/she gets.

someone who tries to pick holes in your logic.

these people have no lives, all they care about is looking smarter than others because they think it makes them superior.