Simple. What pokemon do you use that is out of the blue and for fun or to catch your opponent off guard? I follow this:
When I'm in a fight, and I'm about to lose I just use my Lvl 100 shiny Glaceon and then I win xd There are so many pokes I like though.. Can't choose ^w^
Oh, easy. Uxie. I keep a Uxie on my team for VGC Doubles. Then everyone I face is like "oh why would you do that Cresselia outranks it in every way" Then I start using Yawn and everyone realizes that it's about to be the trolliest match ever.
Sleep Talk is fairly common for Rest/Talk Pokemon, but Yawn still does the required thing to others. I use it as more of a psychological method of battling - they think that since there's a chance they can save their Poke from falling asleep, they switch it out. I get a free hit on whatever they switch in, or the other Poke involved if necessary. I've also never seen Uproar used in competitive. It may be common, I don't know. But I have yet to see it.
Hm, it's been a while, so I don't remember. I believe I used to use Thundurus or something of that sort.
When I had my Y, I used to use my Aggron with a Life Orb, when people never expected it, I'd use Heavy Slam+Autotomize and nearly or possibly OHKO them, due to Aggron's weight and the Life Orb increasing the damage the opponent takes.