Mega Abomasnow, do you need it? For PvE, even as a mega it’s not the biggest help in raids, like any mega, it helps boost like attacks for your group. For PvP though? It’s very strong in both great and Ultra league, double weak to fire doesn’t hold it back, when it works, it’s spammy and annoying to deal with if you aren’t prepared.
What to use? There are a lot of community day pokemon on this list, but we just had some of them recently. The top legendaries have been in raids recently, and when all else fails, try for a random flareon evolve.
Go to the official counters page for a complete list including Shadows and Megas to see how your Pokemon perform.