cheap out on an attenuator and you risk destroying your amp, don't cheap out on attenuators.
I'm with the 'Best Attenuator For A 100 Watt Amp Is A Second, Smaller Amp' mentality.
It was my one and only thought when I saw the thread title. It's been my thinking since I was a teenager in the 1970s and big amps were the norm. (Heh, we didn't have attenuators back then, or even master volume controls. Let each amp be what it is and do what they do. If you needed a big amp, you'd also need something smaller as well).
This /\ You can't use an attenuator to make a 100W amp in to a bedroom practice amp.