That's a great question, but sadly there is a little more information that's needed. What size salmon/steelhead will you be catching and how big is the river that you'll be fishing in? Here in NY, a 6-7wt will work for most Lake Erie steelhead, but the steelhead in Lake Ontario are a lot bigger and an 8-9 weight would be much better.
With all that being said, an 8 or 9 weight should work for most situations with large king salmon needing up to a 10wt. I use an 8wt for most of my steelhead fishing and as long as you don't mind playing the fish a while, you can land just about anything.