Washburne
Protector of the key.
Alyson's a decent example since we actually have seen Willow portrayed by another actress. And if Riff was the 2nd best, I'd say, no, the show would not have made 7 seasons without Alyson. Though in fairness, I'd imagine Amy Acker could have done a fair job as Willow.
But what's to say that Riff was second best? If you can imagine an actress you know doing a good job there's a chance there's someone you don't know or didn't think about, or was too "big" to even be considered for the part that could have done a good job as Willow as well. But if we for some reason state the fact that Riff was second best of course I agree that no one could done the job as good or better than Alyson. The problem is that you'll never get me to agree to that statement. You never know how good an actor/actress will be in a part until later.
Spike is a good example of that. If they'd known how good and popular he's going to be they had never thought of him as just monster of the week. The same goes for Angel. He was never intended to stay for long, be a vampire and definitely get his own show.