Jay, I use 2 different ones depending on the size flies I'm tying
Generally for large saltwater flies or other stuff using bucktail, I'll just roughly even it up by hand rather than stacking to avoid a paint brush look with ends that are all perfectly even. But if I do want to stack bucktail for some reason, I use a brass Griffin Large Stacker. This has a tube diameter of about 1" and is 3" deep and runs about $20
But I think this model has been replaced by the Griffin CHS Magnum with extra tube about $14
Griffin also makes other sized stackers, as well as an 'adjustible' stacker that sounds good in principle but I've never used it and don't know it's range of sizes. It does not look like the tube is deep enough for bucktail on large saltwater flies though.
For smaller stuff like wings on trout flies I use a cheap aluminum double-sided import with tubes of two different lengths-- one for small flies and one for larger ones. If I was to buy another one of this type, depending on what size flies I was tying, I'd probably go with a brass double sided one like this Renzetti Double Ender Hair Stacker (about $20). It has one brass barrel 1/2" diameter and 1 3/4" deep and another 1/4" diameter and 1 1/2" deep
or for smaller trout sized flies the Renzetti Midge Double Ender with one brass barrel 1 /8" diameter and 1" deep and another brass barrel 1 1/8" deep also 1/8" diameter.
The Griffin and Renzetti's all have rubber padding to help dampen noise and reduce scarring table tops. Although single tube stackers are a bit less at $14 and come in different sizes, the double ended ones are a better deal since they'll allow you to tie a wider range of fly sizes if you tie a range of smaller stuff like small sparkle duns, elk hair caddis, medium sized stuff like Wulffs, Trudes, and some larger dries like Stimulators.