Every once in a while we see sites and tools in the TF2 community that we think are cool. The Crate Unboxing spreadsheet is one of them. It gives actual values (in scrap) of the average unboxed item and even factors in the cost of the key to unbox. Very nice.
As of today (it gets updated once per week) the best crates to unbox look to be Salvaged Crate #30 (which is a no-brainer) and crates #13 and 14. Also the current crate #37 is near the top of the realized value but that is more a function of it being new. The coveted #19 crate is also up there.
Overall a smart little tool. Link here. FAQ is here.