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You can't really answer this kind of question with a "I like 'insert provide name here'" type answer because like so many things it is a balance and the reasons for choosing a payment processing solution tend to be complex.
The most important factor in choosing a secure payment clearance service (the people who will connect to the banking networks and clear the money for you - will refer to them as SPCS) is how many widgets will you be selling at what cost. The pricing models of all the SCPS providers is based around this equation. This dictates the economics of using the service, which is nearly always the most important factor.
For example, in the UK securetrading.net have a large annual fee and high minimum transaction values (been a while since I've seen the exact numbers and they don't make it immediately obvious on the site, but this is for illustration only anyway) making it one of the most expensive solutions to use if you are selling high value low volume. Most smaller clients will fall into this model. High value is really anything over a couple of dollars. Low volume is typically anything less than tens of thousands of units per month. However, if you are running a donations service in the aftermath of an international environmental disaster (relatively low value very high volume) then they become one of the cheapest.
Factor in to this the setup costs (relatively high), and the cost to tie the service into the site (in SecureTrading's case it's very easy to do, but still a lot harder than adding a PayPal button) and you start to build up a true picture.
On the flip side, a service such as PayPal has very low setup costs (no fee to pay, and trivially easy to integrate), but relatively high transaction costs. It is great for high value / low volume transaction
esellerate
if it is Digital stuff that you are selling, I recommend http://www.esellerate.net/ . they have nice support for website payment, delivery of serial numbers upon sell and even have API so you can integrate the buying process into your application in case it is a desktop application.
I've looked at WorldPay and SecPay in the past; you need to know your onions to use them competently, I think - if you want really nice integration, at any rate.
Try AlertPay, they have very competetive fees.