SPONTANEOUS !!!
which song would you want to play with someone never the less how!!! is it the guitar part,,is it only the singin,,is it harmony(back vocals) is it the bass,,is it drums,,, well i hope you got me,, on this moment i don't know(it came spontaneous) so don't think really,maybe just the song and a bit why..
Maybe not so much a song. More of a jam. One of my favorite things is to just choose a chord progression and a friend and I just trade it back and forth. One plays a solid rhythm while the other one improvises all sorts of different leads, arpeggios or whatever over it. We just go until someone loses it and we crash. Great fun!