Depends - I like 19. Some like more and have seen gains at 21. Some have lost power above 18. It really cant be determined which number will work "best". 15 to 17 is average and you won't have problems.
But if you want to squeeze out every last hp - use a dyno. But what I will say is whats on SER.NET. Always use 89oct fuel on anything above 15.
I lost gas mileage and power at 19 and hit max power at 17 with 30+mpg...I thought 19 was with the JWT ECU and 92 octane only...I could see how it would depend on the engine, piston ring left, carbon deposits, dirty injectors, plugs healthy, etc...we did my tuning on a dyno, set it at 19, then I drove it for a week and went back for final setting at 17. The power loss was very slight and it happened that I had an error so the timing kept retarding anyway. We adjusted the TPS and this set the problem straight, but I still lost hp at 19 BTC and got horrible(23-26 mpg). If it's at 19, make sure your plugs are good as well.
My understanding has always been that you have to be carefull how much you advance timing with a JWT ECU because it advances 2* over whatever you've got it set to. So you actually have to set it down 2* lower than what you see your maximum power at.
I'm not sure what number, they were iridiums and that was 8 months ago. I was thinking about it today and I did all of the timing/plugs before the mods(intake, header, exhaust). Maybe with the engine breathing better and 142hp, 128 lb-ft at the wheels, I could take the hotter Denso's or even 19 degrees timing...just a thought. Anyone know for sure?