Structures, water, computers, languages and people (not necessarily in this order)

Preferred geometry file for FEA: IGES, STEP or SAT?

IGES and STEP are the widely used neutral CAD formats and accepted in nearly all software packages.

IGES or IGS (Initial Graphics Exchange Specification) is the first CAD data exchange standard developed and it is capable of exchanging only geometry and topology information between different CAD systems.

STEP or STP (STandard for the Exchange of Product Data) enables storing, retrieving, sharing and archiving member data (f.ex. profie sections, material properties, etc...) throughout its life-cycle among different databases and STEP has become the industry standard for Data Exchange.

For Finite Element Analysis (FEA), preference for one or the other depends upon several things including:
1) the source CAD that is used to create the neutral format.
2) the target CAD, CAE or FEA package and the use for which the file is intended.
3) the kind and complexity of geometric elements included in the design (e.g., prismatic elements, splines, surfaces, etc.)

In IGES the output is in lines and surfaces while STEP file keeps the assembly hierarchy and output is a mixture of solids,volumes and surfaces. For FEA purposes IGES may be enough as you don't usually need solids/volumes. IGES works ok for importing beams and sometimes works well for plates.
For those that work in the 3D solid modelling world, STEP format may be preferred as it retains the solid geometry and brings the part into the 3D modelling software in solid format. In general STEP is also more robust than IGES for plates and shells imports.

ACIS SAT is very similar to STEP. The kernel of many AutoDesk and Dassault products is partially based on ACIS. For example Abaqus uses ASICS (*.SAT) as it's native kernel so I usually start there and then go to STEP and IGES would be the last choice.

Stupid as it may sound, I generally request more than one format and a screenshot image of what it should look like.