SquashGalaxy's #1 recommended string is the Python Atlas Deluxe 17g Black. It's a soft core multi-fibrous string, meaning it is manufactured with thousands of individually wrapped strands and as you play you will develop a fray patch and gradually wear away strings. The thousands of fibers give added durability to the string and the soft core allow the string to stretch giving more power to your frame.
If you are not a fan of multi-fibrous string we recommend the Ashaway SuperNick XL. This textured string gives great ball control while still having the durability you are looking for. Both of these strings play equally well but if you are a string breaker the tecnifibre 305 might be the better way to go.