Warframe as of update 33
Order within tiers is alphabetical. Factors taken into account are DPS, survivability, utility, support and a bit of personal bias (all tier lists are biased, deal with it). Some placings (Voruna, Vauban, Nyx) not entirely sure of due to lack of experience with them.
Credit to https://www.warframe.com/game/warframes for the images.