I'd wager that most people here haven't used one of them, nonetheless all three, so you're not going to likely get quality results. Besides... "best" is relative. It all depends on your goals. If all you care about is irrelevant backlinks then PRweb would suffice. If you care about real coverage, the above-mentioned can work well, but generally manual distribution will be even better, as is submitting for free to AP and getting it picked up.