In my opinion it depends on where you are traveling to. I'd say that to any western\developed country a credit card and some cash would be enough, as virtually any kind of business will accept it, and it would also be easy to find an ATM.
The problem is with developing countries such as India. Most shop and merchants won't accept credit cards, and it's impossible to find ATM outside big cities. carrying cash is also not so safe as there's high rate of stealing from tourist. For such countries Travelers Checks are the best option, as many businesses, even on rural areas accept them, and they are insured.