I don't know about OpenCart but I guess that they are pretty much a clone of what is considered a versatile shopping cart.
What is the best?
The best is the one that suits your requirements and no one shopping cart system will do that when compared to a purpose designed cart system. Yes, there are a lot of sophisticated shopping carts out there and if anything they are all over-designed because they all have one thing in common, and that's to appeal to as many different users and tasks as possible.
And that's why I write my own. About a month a go I revised a cart solution that I first wrote about 10 years ago. Over time it had been used on several different types of web site and today incorporates many features like newsletters blastable by category, "have your say/comment" options, eBay style pre-purchase messaging, all of which can be admin moderated. So for a custom job I simply collated which functions my new client wanted.
Now I am not selling my cart to you and you will never get it without being a hosting client, but the point that I want to make is that you just cannot beat purpose designed applications.
Oh yeah... this topic is about SEO. Well the difference between [manually entering page title, meta description and keywords] and [letting the shopping cart system do it for you] can be nothing, zilch, the same... if you realise how your shopping might work.
For example your cart will be adding the product title to the page title. Then it might either add the product summary to the meta description tag or use the first 160 characters from your product description. The keywords you will probably type in on the product's settings page anyway. Other things like alt tags on product images will probably come from product title also.
So if you are careful about what you type in for your product's title and summary you have it sewn up already, and no more to worry about. Or do you? Has the shopping cart maker done the all that he can? For example has he also included the category in the page title , and also included the category description in the meta description tag? Note that I mentioned category "description" and not category "name". If not you can add these items to your cart templates for both category and product pages.
And now that you have properly organised your meta tags and alt tags, its now up to rabid search engine to penalise you for being so professional :-)