PowerDVD - because it's the one I've always used, and TMT has either not worked or crashed (I mean not even bluescreen, but caused the computer to reboot spontaneously) when I've tried it.
Right now I've got PowerDVD 7.3 and PowerDVD 9 (both "Ultra" versions) installed on my HTPC, so that I can watch the few HD-DVDs I still have. Once I have replaced all my HD-DVDs with Blu-Rays, I'll drop PowerDVD 7.3.
For folder-based DVD playback, I used to prefer VLC, but it has some strange problem on my computer regarding audio dropouts that the VLC people have never been able to figure out - a few of us Windows users have this problem. Since the problem appears so rare (and is probably caused by my sound card driver - a high end one (HT Omega Striker 7.1)), they have little incentive to debug it, and they have recommended I use Media Player Classic - Home Cinema, which does work. I prefer the default keyboard shortcuts for VLC, but those audio dropouts are really annoying.