People on here will suggest of sorts of things, but whats suitable for you depends what you need it to do. If you need it to plan PCBs for instance. Also, on your budget.
Eagle. At least it was when I looked at them. I'll probably pay the price for the full version pretty soon; rules-based PCB design is nice to have, and the schematic capture and library editing work pretty darned well.
Eagle is very good to make PCBs, but if you are after schematics an old fashioned pencil and paper may be your best bet....provided they are for your use and not to impress other people.
This topic has been discussed so many times over here. Please use the search function to find if answers to your questions already exists before posting.