Up until recently .X + in-dbp rendering engine.
However for a new project i'm working on, probably work on a new format; Currently i'm working with DBO for stuff, which is proving some fruitful results in terms of pure speed.
Hopefully with Service Pack 6.0 there will be C++ access to the DBO format, I've already been using the low-level mesh commands thanks to IanM's update for his Interface.
The new project means alot of new features must be developed to get it running at a reasonable speed.
Currently BSP just isn't worth the hassle unless your making a pure FPS.

Athlon64 FX-51 | 1.5Gb DDR2 PC3400 | GeForce FX 5900 Ultra 56.60 | DirectX9.1 SDK | Audigy2 | Windows XP 64-Bit