Nextag and Pricegrabber are both US companies that are not as dominate in the UK so there customer service is often on a different time scale and is focused somewhat towards the US market.
Shopping.com
PriceRunner
Kelkoo
Shopzilla
Pangora
They are the key players within the UK to get with for everyday products and vdeep.com if you have any adult products.
Nextag
Pricegrabber
Caio
Would be next on my list followed by some of the smaller engines the reason being that those digging for a bargain will check out the latter ones listed, and often the other smaller ones can just appear in Google for some searches.
Also shopping.net can be useful I keep reading but I dont know much about them.
I noted someone motioned receiving only a single click from Ciao, while a single click is unusually it must be noted that lion shares of Ciaos revenue comes from distributing surveys and that the shopping comparison side of things is very much an add on and they therefore are not always as popular as the other sites, but I must say I am surprised at only one click a day.
It was mentioned that Kelkoo is cheaper but is several times bigger. When looking at each of the comparison sites you need to consider there business model and the type of consumer that uses them predominantly. In my experience from talking to people and research Kelkoo will drive you more traffic than any other shopping comparison site in the UK but does that make it the best??? No. Kelkoo is very much like a Google of comparison engines turn on the tap and the traffic will come, but will they come and buy that is the question.
The fundamental question is ROI, thats what matters. Each comparison site will have categories they are strong on. I would personally list with as many as you can providing you can get around the initial listing fees, and see how each one converts for you, do not always be put off buy high the CPC rate, because if there traffic converts well thats the important thing and the comparison site that does not work well for one person may well work well for you, often it can depend on your products. Remember a comparison site can only charge what the market will bear, if the charge to much and you fail to make money you will remove your products and they will lose you. They need you as much as your need them.
Also as I said in a pervious post maximise your use of the data feed, also if you can install or monitor your ROI with them as best you can, preferable install there ROI code if they provide it, also if they provide code to encourage users to review your site maximise your use of this to (providing you know you give customer service and sell good products). Also ask users where they found your site, often users will find your products on a comparison engine and come back at a latter date and purchase not crediting the comparison engine as providing that sale, this can be doubly important for those sites with telephone numbers on the front page, have your sales team ask them how the found you.
Sorry for the length here I just thought I would add a bit and then it became a lot I hope it helps.
I would be very interested to hear what conversion rates you or anyone else is receiving form any of the shopping comparison engines.