For an objective evaluation of conferences, we need an official third party whihc evaluates all the conferences, thus producing a credible classification (A, B and C or impact factor calculus).
Just a short answer that might help: Thomson Reuters offers the Conference Proceedings Citation Index- Science (CPCI-S) and the Conference Proceedings Citation Index- Social Science & Humanities (CPCI-SSH). You could use these as a basis for a ranking (e.g. by using the impact factor). I hope this helps a little.
The first link is a web page that includes a list of ranked conferences and journals, while the second link is a file that includes a list of journals taged by impact factors according to some measures.
you can also take a look to a more recent intiative by two Italian and a Spanish Computer Science societies where they have gathered and combined three different conference quality rankings :
I would recommend this website http://www.guide2research.com/. It provides updated rankings and ratings (important ones) for all the conference and journals. I found it quite relevant to my field of research (Computer Vision, Machine Learning). It includes different independent accreditation and also has good filtering options, so you can set different parameters to check the reputation of the particular conference or journal.
You should be aware that most of conferences appear to be scholarly conferences, but are in reality an exploitative means used by predatory publishers and conference organizers to make money in the form of registration fees.
Conferences dont have an impact factor. To rank a conference you need to look at it's rejection rate, the higher the better. Also the chairs and the conference committee are a good indicator of the conference quality. The audience of the conference is also another factor. High quality conferences attracts and is built on high quality research.
Microsoft Academic's field ratings for conferences. Microsoft Academic describes field ratings as follows: "The field rating is similar to h-index in that it calculates the number of publications by an author and the distribution of citations to the publications. Field rating only calculates publications and citations within a specific field and shows the impact of the scholar or journal within that specific field"