I listened to both the VTF-15 and the ULS-15 in the same room at Hsu's headquarters.
1) I don't think you need to worry about the amplifier power. With a single VTF-15 running its SPL output was amazing. My pant legs were actually vibrating. (I don't remember which movie Dr Hsu was playing).
2) I don't think there could be a better value for overall Home Theater performance where max SPL is a primary consideration.
3) Despite my praise for the VTF-15, I left Hsu headquarters with Dual ULS-15s. They have a tighter, more linear sound (even vs the VTFs in plugged mode). They have more than enough output for my home theater. But, they won't put out as high an SPL level in the mid/upper bass as the VT-15.
4) So, you will hear virtually nothing but praise for the VTF-15s (here and at. But, you will also find posts from Pete at Hsu (and others) that discrimate the differences between these two Hsu subs. I've never heard the Rhythmik FV-15HP that's about to have its review posted here. But, that might be another option.
There ain't a single answer for "the best". But, like others have posted, I can't imagine you'd be disappointed with the VTF-15 - its owners are overwhelmingly positive in the various online forums.