I prefer 63/37 solder with rosin core. Usually I get mine from MG Chemicals. Frys carries it if you have one near you. 60/40 is good too, it's just not as consistent as the 63/37

Silver solder is technically cleaner but it requires more heat (not ideal for sensitive electronics) which means you may need to hold the iron on the solder tab longer than usual. So I've stopped using it and went back to lead solder

As far as bullets, I use TQ 5mm and 4mm bullets for my batteries and I really like Castle's 4mm bullets for motor wires if you want the option of disconnecting the motor without desoldering. And the added resistance hasn't hampered my motor's performance.