I use various threads. I like UTC 140 for big streamers. Mike Schmidt got me hooked on it. It has nice stretch and is strong.
I use Danville 3/0 monocord for many uses. I use it on larger dry flies. It is a strong thread. You can use it on big streamers too. It doesn't have the stretch that 140 has, but I still like it.
I like UNI 6/0 and 8/0 for smaller dry flies and small flies in general. Great general purpose thread for trout flies.
I have also tried Veevus for trout type flies. It is very strong for as thin as it is.
Just my 2 cents.