I think being comfortable with whatever position you choose can make all the difference, and comfort can depend on a few physical limitations (like flexibility, size difference, general stamina, etc). There are quite a few examples in the Positions of the Week topic to have a browse of for some inspiration, and they tell you a bit about what each is good for, as well as including a few tips to tweak them better to your preferences. 
And there’s the 
Oral Sex Superbowl for a few extra oral tips. 
People like different things, so it’s hard to give a definitive ‘best blow job’ breakdown, but there are plenty of topics discussing it if you have a Search.