What are some metrics to rank conferences?
There are multiple rankings for Conferences:

H5-index:
H5-index is the h-index for articles published in the last 5 complete years. It is the largest number h such that h articles published in 2010-2014 have at least h citations each.
(Available at: https://scholar.google.com/citations?view_op=top_venues&amp;hl=en&amp;vq=eng).

ERA (2010):
This conference ranking has been created as part of the Excellence in Research in Australia (ERA). The ranking was created by Australian deans and the Australian Computing Research and Education Association of Australasia (CORE). The rankings range from A (=best) to C (=worst).
(Available at: http://www.conferenceranks.com/).

Qualis (2012):
This conference ranking has been published by the Brazilian ministry of education and uses the H-index as performance measure for conferences (Computer Science). Based on the H-index percentiles, the conferences are grouped into performance classes that range from A1 (=best), A2, B1... B5 (=worst).
(Available at: http://www.conferenceranks.com/).

CORE (2014):
The CORE Conference Ranking is an ongoing activity that provides assessments of major conferences in the computing disciplines. The rankings are managed by the CORE Executive Committee, with updates processed from time to time by a subcommittee established as needed. CORA metric for each conference and workshop is calculated by a mix of indicators, including citation rates, paper submission and acceptance rates, and the visibility and research track record of the key people hosting the conference and managing its technical program. Based on the CORA percentiles, the conferences are grouped into performance classes that range from A* (=best), A, B, C, Australasian, and Unranked (=worst).
(Available at: http://portal.core.edu.au/conf-ranks/).

Update: CORE 2017 is recently available