just made a post about this but my long time friend and fellow coach just published this short book breaking down his tried and true offensive playbook and more importantly how to teach it to the team. id strongly recommend checking it out. :)
Hardest flags to pull are Sonic 'Flag-a-tag' with the mushroom cups. However, most leagues have banned these. There are still some leagues/tournaments that allow it. They are hard to find since they are no longer made.
Here is a link on amazon for flags: http://www.amazon.com/Flag---Tag-Sonic-Scarlet-52-inch/dp/B0010R5IOU/ref=sr_1_1?ie=UTF8&qid=1454511216&sr=8-1&keywords=sonic+flag-a-tag
In my experience, after you finish playing -- it is best to pull the flag off the socket so it doesn't stay stretched because over time it will loosen up and the flags have a hard time staying on.
Hope this helps.
LV Gridiron uses these:
Cheaper than the spotters and squishy enough where nobody gets hurt if they step on them.