I agree with tedshuck: I ALWAYS use a loop knot for single flies. The non-slip mono loop knot he mentions is a great knot for smaller tippets, and I use it for my trout flies, but for larger S/W stuff (you didn't say, but this is the S/W section) I like the DOA knot:

http://www.doalures.com/uploads/Docs..._Knot_Page.pdf

It's not as strong (%) a knot as the NSML, but is a lot faster and easier to tie and on 15-25 lb test line, knot strength just isn't that critical because you'll never break it with the pressure you can apply with a fly rod. With a little practice, you can tie it with so short a tag end you won't even need to trim it.