It depends in what context. If you're looking at viability of Pokemon for your own team, generally I wouldn't use anything below B rank unless you have a very clear reason and it fills a niche that you have identified, for example using Mega-Gallade over Medicham, because the use of Knock-Off and 10 extra base speed is necessary for some specific threats.
If you're wondering because you're trying to beat the "relevant" mons with a team you're building , I'd recommend checking out the
usage stats here instead. You can see that some Pokemon have much higher usage than ranking (Mimikyu) or used much less (Reuniclus). You can see exactly how often they are used and therefore their relevancy.