If you are fucking a woman a King, so you can push her to the side when you are done.

If you sleeping with your homie, a Bunk, so you can wish him sweet dreams and if you have a bad dream he tell you it ain't real.

For yourself, a double. For a couple a regular king, not that skinny California King shit.

Eastern king but train her to stay on her side otherwise you might as well get a full

My wife and I fit comfortably on a queen because we aren't far slobs.

Four Alaskan kings butted together.

I'm 6'4" and married, queen for me. Kings are too big (though I think a UK king and US queen are similar)

When I was single, a double used diagonally worked, and once I found a tall twin that was cool, till I tried to find sheets (pre-internet shopping).

Best is a function of what you need. Your best and anyone else's best probably won't be the same.

I'm short and cheap, I sleep on a twin

Hammock under the porch roof.

If you're single, sleep on the ground with a thin mat or get a cot. Better for your spine. Your muscles are constantly activating when you're laying on a cushy mattress. Vs, letting your spine and bones and tendons etc completely relax on a stiff surface. Some of my friends go no-pillow, but eh, I still find it more comfortable than training natural sleep positions.

Queen. Big enough for two, doesn't take up a shit load of space, and if you're alone you always have the other side of the bed to roll over onto.