Highest octane available wherever I'm filling the tank...
...same here.
I fill up at the local BP, which has 93 octane, and a separate hose for each grade, so I don't get a half gallon or so of what the guy before me was buying with the single hose...
My gas mileage goes for sh*t big time when I don't run high octane gas. I'll also add, that the better the quality the gas (I'm not talking octane here) the better the bike runs. My bike runs noticably poorer on Shell gas than it does on Esso gas. It has something to do with the additives that the producers put in the gas.
Grudgingly, I use 93 Octane, since I can't find any "real fuel" that's not racing fuel at $10 a gallon. How I miss Sunoco 260 at the regular pumps for a few cents more...
****!!! My bike pinged so badly in stock form with 91 octane, I can't imagine what it would have done on 87. I will always go for the highest octane available, and that's 91 in my part of the country.