There are so many to choose from! so there may not be a specific 'best' one in general - it may depend on what you are trying to predict using QSAR methods.

I find the Balaban topological index a good one:

A.T. Balaban, Highly discriminating distance-based topological index, Chemical Physics Letters, 89 (1982) 399-404.

Additionally there are the Kappa shape indices (Kier and Hall) which are also good shape descriptors too

Hope that helps!

Danielle

You can calculate a lot of molecular shape parameters using on-line computations at www.chemosophia.com such as molecular volume, surface, non-polar, positively and negatively charged areas, etc., etc., etc. (select "Chemosophia Elastic Calculations" in "Single molecule calculations" or "Large organic and elementorganic molecular systems" services) now for free. Additionally, Elastic calculates a lot of physicochemical properties. Besides, you can calculate hundreds descriptors related with molecular shape using "Calculation of descriptors" software in the same services or in "Calculation of descriptors and molecular superimposition" service at www.chemosophia.com .