What is the best program for designing circuit diagrams
If you want only schematics entry and free, then I think you only have 2 well supported options: Eagle or LTspice. Free Eagle is an entry level full eCAD program, so it also supports schematic entry and does it well with a huge online library.

LTspice is the other option. It's a simulation program but simulations need schematic entry too. LTspice probably has a slightly easier learning curve. YMMV.