I'm not sure there's a simple answer to your question, since obviously Ben's response was a tease.

I suspect you meant reliable, trouble-free hardware? There is no “best”, however, I would recommend using some sort of unix-y solution, since that's where Houdini started, and there always seems to be little things that crop up with Windows. After that, if you're looking for reliability - well, I'm obviously a Linux fan - we use them and they're great. Cheaper than SGI, more production-friendly than Windows, and with (arguably) more software available than Solaris. But then I'm biased.

We've had very good experiences with Geforce cards and Nvidia's Linux detonater drivers, however you will always have little glitches if you use the consumer based gaming cards like geforce4 TI. They're *usable*, but you get little problems. Others hack their geforce 3's and are very happy, but for production we use geforce4 quadros…we're happy with those. We run Redhat 7.3(recommended over 8 and 9 for production - in fact 9 doesn't work at all).
Apart from all that, the latest ATI cards are supposed to be quite good - previous generations either suffered horribly or lacked decent Linux drivers - but I've heard from more and more people that they're much better now.
Otherwise - be sure to check out SESI's recommended hardware on their web page - it's a *little* behind the times, since things change every month in this biz, but it's a start.
Hope this is a little useful…
Cheers,
J.C.