Automatically generated combination pages should not be presented to search engines.
- There is going to be a lot of duplication
- Having Googlebot crawl a large number of such pages can decrease the crawling of the rest of your site
- There usually isn't any search volume for comparison terms
Comparison features can be great for users on your site, but they are not a good way of attracting users from search engines. Use robots.txt to disallow search engine bots from crawling all those pages to begin with. If you do so, you won't have to worry about canonical tags at all.
If you find there is search volume for some combination. Spend more time on it. Write a more in depth article. Include the automatically generated comparison table.
There are some exceptions to this advice. For some product types users do search for comparisons. If you can build a good user experience on each comparison page you can get away with a lot of duplication in the text on the page. You would have to do a good job of generating unique page titles and meta descriptions for each page so there isn't duplication there. You would also want to make sure that you link prominently to the most popular comparisons (for example from the home page). You'll probably find that 20% of your comparisons get 80% of the interest and that 50% of comparison pages aren't even used at all. Advanced strategies rely on link sculpting to the popular ones and disallowing crawling and indexing of the unpopular ones.