The person I'd choose is my university friend Laila, who I studied architecture with in Lahore. We've done a few trips together, but the best was a week in the Hunza Valley in the north of Pakistan two summers ago.
We travelled up by road, which is a long, winding journey, and stayed in a small guesthouse in Karimabad run by a local family. We did quite a lot in a week. We walked up to the old fort, took a jeep to Attabad Lake, which is an unbelievable turquoise colour, and spent one whole afternoon at a village apricot orchard, helping the owner's children pick fruit.
So why is Laila such good company on a trip? First of all, she's flexible. On the third day, a landslide closed the road we'd planned to take, and while I was panicking about our plans, she just said, "Great, now we have a free day," and found us a walking route along the river instead. It turned out to be the best day of the whole trip. Second, she talks to everyone. Because of her, we were invited to drink tea with an elderly man who told us stories about the valley before the road was built. And third, she's funny without trying. She kept a diary of ridiculous things that happened, like the goat that followed us for an hour, and read it out every night.
I think a good travel partner turns problems into stories. With Laila, even the bad moments become memories you laugh about later, and that's exactly what makes a trip worth taking.