I can't say whether Guam is best BB1, but from my experience of playing US and UK BB1s, it is my favorite to play.
It might not be the best in BB1 vs. BB1 fight, but in a GB where any BB1 will be outranged anyway, being able to go 45 knots is a great asset.
It allows me to get away from higher level bbs easily, and it lets me get in range quickly and get some attack.
I've played both Guam and Tenn41 with a high level crew, and I feel "safer" and more useful in my Guam than I would ever feel in a Tenn41. Sure, I'm outrange and outgun, but my speed allows me to stay back most of the time and wait for right moment to rush and get some attack. With the Tenn41, I'm still outrange and staying back, but I cannot easily move in to engage enemy if opportunity arises. Worse, if I'm being engage by higher level BB, I cannot run away in a Tenn, I have to rush and hope to get in range before getting killed, but that is hard considering most high level BBs are faster than me. So Guam is better in the sense that you are not both out ranged and out speed. You can at least run away.
Not to mention AA, if you can't be useful as a real battleship, at least you can AA to help your team. Not many BB1s can do that.