Which is the best / cheapest / fastest payment gateway?

The answer really depends almost entirely on your needs. Fees usually vary based on your volume or line of business. Some gateways have low or free monthly rates, but higher per transaction rates, a good fit for a casual user. Other gateways have higher monthly fees, but cheap transaction rates, making them better for high volume customers. Still others have fixed transactions rates, a real bargain if you're charging high value items.

Finally, advanced features such as fraud detection, recurring billing and storing credit card numbers differ between gateways and may factor into your decision, especially as support for these features is added to Merchant for Android.

would be my recommendations for most users.

Most gateways can be up and running in 24-48 hours.

The only gateway provider that I don't recommend is PayPal.