Someone hands you a guitar and says "Play something.."

PhilW1

‘So What’ ..Anti Nowhere League/ Metallica 

Don’t get asked again, especially at kids parties 

p90fool

I was playing at a biggish festival in the South of France a few years ago and there was an after show party with all the musicians etc in the gardens of the organiser's beautiful mountain retreat.

There were some fantastic musicians there and a couple of acoustic guitars were being passed around among some of the continent's best blues and jazz rock artists to great appreciation from everyone, then someone's roadie was given a guitar and he sang and played Won't Get Fooled Again.
It was more intense and passionate than anything else there and you could hear a pin drop until the end, when the place erupted. 

It doesn't answer the question, but it's always put informal gathering music into a kind of perspective for me. 

Dominic

"You've got a friend in me ".......The Toy Story theme by Randy Newman
I'm an absolutely terrible singer so I like the fact that you can half -speak the words 
It rings a bell with adults and kids and is quite catchy
Samba always impresses ..(they invariably hand you the kids nylon unloved guitar ) so Girl from Ipanema 
Then,once I have their attention I subject them to something a bit more mature like 30 minutes of Villa-Lobos or Segovia (if only )

Dominic

Another half-spoken one is "Mamma told me not to come " ...Sterophonics /Tom Jones version
quite relevant at a party and fairly easy

Octavio

Thanks for all the suggestions. I might be able to perform some of these and look at myself in the mirror afterwards @Danny1969 great playing! 

Fez

All along the watchtower, Leopardskin pillbox hat, Cocaine blues, if there's a capo Goldwatch blues.

BigLicks67

If I'm singing & no capo, Buckets Of Rain, Blackbird, Don't Cry No Tears or Behind Blue Eyes.
No singing & chord melody then Darn That Dream or Georgia on My Mind.
If the room is full of cork sniffers then I'd do Bach BWV1007.

Philtre

Sleep Dirt.