In the past six years of flying, I've rarely had anyone request anything other than the standards (Bloody Mary, Gin and Tonic, etc.) except for:
Vodka Collins - easy but we don't have collins mix on the plane - used 7up and lots of lemons.
Manhattan - my personal favourite, but no sweet vermouth on the plane (let alone a decent maraschino) BUT I improvised and this guy was really pleased with my faux concoction. Guess my practice at home pays off....
Absolutely the WORST drink requested (which I happily supplied - it was actually so bad sounding it was funny) was Diet Coke and Red Wine on the rocks. To this day, I can't imagine....