i've shot buckshot throught just about every choke from improved cycl all the way down to extra full turkey (.665) with success, it all just depends on how far your trying to shoot and what size shot your using. buy if i had to choose only one size choke to use then it would probably be modified. i've been able to produce some serious deer killing patterns with a .670 turkey choke and 3 1/2 inch #4 buckshot out to around 60 yards.
best advise just experiment and have some fun
Good Luck Zack