I like whatever I can get! I've been back and forth on this question. I might lean a little toward .999 just because I like that a 1 oz round is 1 oz of silver. If I see 5 ASEs, I don't have to break out the calculator to see if I'm getting a good deal or not, like I would have to do with 5 90% quarters or halves. Not a big deal, but I also just like the shininess of .999.
Personally, I also like 80% Canadian - I like the coin designs and I like the fact that all denominations are .6 oz per 1$ face.
I have no idea what my actual percentages are, but, just for fun, something like this would be my preference:
60% .999 (50% government rounds/10% generic rounds/bars)
25% 90% American
15% 80% Canadian