Everyone has their own opinions on the best soil... There is a section here on the org just for soils and additives...
Banana Plant Soil, Additives, and Fertilizer - Bananas.org
The soil I use is the Cactus, Palm, and Citrus soil that you can pick up at Lowe's or Home Depot... Sometimes I buy the MiracleGro, sometimes I buy the Schultz brand... Mix in a little Perlite, and it has worked fine for me for several years...
I am just a banana hobbyist, though... There are several people here that take this much more seriously than I do...