Proteus 6 Professional in my opinion
Hands Down. Just watch the price tag!
Why? It supports;
- Anologue Circuits
- Digital Circuits
- Every debugging tool you can think of practically, from the basics like AC/DC Amp/Volt meters, to cro's, hyper terminals, SPI/I2C debuggers, counters, logic analysers, pattern/signal generators
- Automatic Component Layuout for PCB Design
- Automatic Routing of traces for upto 4 layer PCB's
Take a look at this circuit I threw together in 5 minutes;