two different answers
DQ has the best variety
Wendy's has the best individual treat, the chocolate frosty
Honorable mention to Arby's Jamocha shake
Wow, Spot on. The only thing I might change is that I think Sonic is in the mix for best variety and their drinks put them over the top if you include those. McD's is still sub-par but is showing some improvement recently. BTW, sonic still has dip cones. Arby's really only has the jamocha shake but it is pretty good.
Also, BK is by far the worst for frozen treats.
Side Note: Just got back from a walk to DQ and I had a Strawberry & Golden Oreo blizzard. Really good tastes like straw-shortcake