Okay, I'll talk about Forretress after all. Forretress has some potential usefulness in OU now that both Aegislash and Mega Mawile have left the building, but I find Mega Scizor to be better. First, Mega Scizor has basically the same stats, just better in every stat except HP, though a 5 point difference isn't really that big of a deal. Second, Mega Scizor has actual recovery in Roost, something that Forretess would kill for, meaning Mega Scizor can take repeated attacks while Forretress only has a few chances to get the job done. Finally, Mega Scizor can actually provide an offensive presence with priority and high power; Forretress only has 90 Attack, and he can't use Gyro Ball without giving up a utility move. From what I can tell there are only two things that Forretress has over Mega Scizor. First, Forretress can set up Stealth Rock and (Toxic) Spikes, while Mega Scizor has no entry hazard moves of any kind. And second, Forretress doesn't cost the team a Mega Slot, meaning you can have Forretress and still use a Mega Evolution on your team. Though honestly, I'd rather just use Mega Scizor.
tl;dr Forretress has some use in OU, but it's tough to not compare him to Mega Scizor. Now that I look back at it, D rank seems more appropriate for Forretress, since even C- might be pushing it, but I'll let others debate on that one.