I've installed a bunch of different carts, and we're using Miva at the moment for one of our sites (3rd party processor that only provides their service through their own Miva cart application.)
Personally, the best cart I've used, hands down, is x-cart ( www.x-cart.com). It is really easy to install, unbelievably flexible and configurable, and has an amazing feature set, including support for coupons, multiple pricing structures, newsletter, member rating/review system, and lots of other add-ins.
It also supports just about every transaction gateway on the planet.
I was worried about support (the developers are in Russia) but have been amazed at how quickly and accurately they've helped resolve problems (which have been very few and far between).
It runs in PHP, costs about 200 bucks, I think, and I had it up and running, with a handful of items, in under 2 hours from install to ready to take orders.
I know a lot of people like Miva, but it seems really cumbersome to me, and it also seems to lack (or else I can't find them) some of the features that x-cart has. Plus, your host has to already be running Miva Empressa on their server, which means you can't just (as far as I know) buy the Miva Merchant script and install it.
Hope that helps.
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