Kinda depends. We're assuming there's a good steady supply of electricity available, right? If it's just me, I'll have my Phil Jones Cub II amp. 110W, two 5" speakers, loud enough for just me, and light enough to carry with my pinky.
If, on the other hand, I want to also use my rig to attract the attention of nearby passing ships, then I'll go with my Mesa D-800 and my BareFaced 12" BB3 cab. That's probably the loudest rig I own. I also have an older model Phil Jones Roadcase amp with 750W and twelve 5" speakers that may be as loud as the Mesa+BareFaced rig, but it's more of a pain in the butt to move around. I get the feeling that on a desert island, that might be important.