Depends on where I am at, what was served, and service.
Formal restaurants that have the service level to match? These guys get 20-25% easy. And the formal type places that we visit on occasion have service staff that are worth it. If they aren't I don't care how good the food is, I'm not dropping that kind of money in the place. There are some factors I suppose, I might go a bit towards the lower end of the scale if the bill is unusually high due to an unusually expensive ingredient, but it's not a huge factor.
But then at the small homey places we go to much more often, it easy to hit 20% or more as well. But then when breakfast for the two of us is MAYBE 15 bucks, it doesn't take a big tip to exceed 20%. Same places for dinner at the same way. A 20ish dollar bill doesn't take a large tip to exceed 20%.
ApplechiliGarden House? 15-18% usually. I'll go over if they do their job exceedingly well, but I have no problem going less for *expletive deleted*it service. I always leave a tip though. Always. Lowest I've left was a penny a person on a couple of particularly bad experiences. But I always leave something so the server knows I didn't just forget.
And service effecting the tip is always based only on the service. If I'm at Texas Roadhouse it's not my waitresses fault the guy in the back screwed up the temp on my steak. Oh and counter service. I never tip for counter service or take out.