For basic hotel buffet breakfasts either $1 or $2 depending on the quality and location. If I was to go to a really fancy place, 10%. Even if they're doing a lot of work, I'm still having to get up and walk around and carry my own plate, so 18-20% isn't going to happen.

For hotel club lounges, $1, more if I'm there for an hour or so. Except at the Sheraton Commander where I'm there often and the very nice attendant works both the 6-10am and the 4-8pm shifts so I give her $2 or $3 even though the food selections are meh.

I don't generally go to buffets outside of a hotel except for a few specific places with friends in other cities (a Chinese place in Cambridge and an Indian place in Mountain View), and there I let them figure out the tip.