For me it’s the two of hearts. You can see how a hand like two of hearts, seven of clubs instantly looks so much more powerful. I'd certainly not pass up the chance of betting on it. Raising... well, not so much really.
Let's get more sensible. Such cards may act as random signals to bluff and as such should not be pooh-poohed. Players use combinations too. Any five and ten is a popular one on the basis that all straights contain one of these cards. So, you're going to hit your opponents with a not-so-well disguised straight.
And then there's that old perennial favourite: ace and... ace. Ha! Don’t make me laugh. The number of times that's failed?