Another vote for Nothern Brewer on the pile.
I haven't tried a great many bittering hops, but it is because I really love this one. It gives a 'rounded' bittering if that makes sense. I just used Nelson Sauvin in an IIPA, but give me NB any day of the week, it is the Swiss-Army Hop. The character that carries through is very pleasant and adds a certain 'colour' to your beer flavour, but I have never heard of another hop that it clashes with.
If I had to pick one bittering hop, NB would be the one.