Some of these "worst hands" are bad in the same way and will lose at about the same rate, so I've called them ties, even when one is a slightly better hand.
1. 2-77-2 off suit is considered the worst hand in Texas Hold'em. They are the lowest two cards you can have that cannot make a straight (there's 4 cards between 2 and 7). Even if they are suited, they will make you a very low flush, and if either pairs, it's an awfully low hand.
Because it is the worst, some players will play it for fun and in online games, it is known as "the hammer."