While I agree with Amir's point that this forum will have a selection bias, considering how most design load sizing is a form of energy modeling, the answer is:

1. is: 1. Microsoft Excel
2. Excel 2. Microsoft Excel, with VBA macros spawned from Dante's fifth level of hell
3. hell 3. A hand calculator where people multiply sf/ton values they have written on a sticky note on their monitor
4. monitor 4. Trace/HAP and other mechanical load calculation software
5. software 5. Everything else is in the noise