The main ( or lets say the official ) format for DirectX is .x which is completely supported by the legacy D3DRM , and also by D3DX set. the X format is optimized for Direct3D, Also there are some tools to convert from 3DS , DXF , & LWO to .X files.
If you want a format that has a standard application associated with it , I think the easier on is LWO , I'm about to finish a .LWO loader/container class if u need it tell me

I've been digging through alot of books on 3D (which are somewhat old) and alot like the Autocad DXF format which they convert into their own format and use. I'm wondering what 3D file format lends itself to DirectX 7 best these days. The DXF format appears to use square or polygonal faces which i assume would need to be converted to triangles for D3D, perhaps there is already a format that is in triangles instead of these Faces that DXF uses? I may be way off, i'm just trying to figure how to get a complex object into a 3D program, any help would be appreciated.

D3DX and IM don't support any 3d file formats, including .X, .dxf and .3ds. There is a IM demo with the Dx7 SDK which shows you how to load and display a .x file and there is also a converter that converts from .3ds files to .x. And you can convert from .dxf to .3ds with 3DStudio of course.

[This message has been edited by simon_brown75 (edited October 26, 1999).]