I really don't see Mega Pidgeot as S-Rank material. If you really want to get nitty gritty with it just look at its stats. 83/80/80 defenses with a typing that leaves much to be desired alongside a middling speed stat and not too much power backing it up. That previous sentence is a jumbled mess of all the foreseeable downsides of Mega Pidgeot so I'll get into them a little bit here. 83/80/80 defenses AFTER the mega evolution. Prior to mega evolving you're left with and even worse 83/75/70 stat distribution. This is pretty poor for something that usually has an argument of natural bulk thrown into why it's so good all the time.
For example, the once touted absolute best Mega Pidgeot set was the whole "Fuck Stall it's lame and I hate it so I'm going to run only Hurricane and a bunch of other shit set", still working on the name. Anyways, it was the Work Up + Refresh set that supposedly ran over Balance and Stall. This set, like any other Pidgeot set, struggles from lacks of opportunity. Pidgeot thrives when it is facing up against passive mons that it can switch in safely on and threaten out. THERE ARE NONE IN THIS TIER. That's a bit of an exaggeration but when you really look at it am I wrong? Defensive Scalds are doing upwards of 30% while threatening burns, Florges' moonblast does almost half and lowers Special Attack, Krookodile's Dread Plate Knock Off does over half, etc. There is nothing for it to come in on, especially pre mega.
This ties into Pidgeot's shitty defensive typing. Normal/Flying is garbage in this tier, lets be honest. You pack an immunity to ghost (Oh Boy!), a resistance to grass (neato!), and an immunity to ground (this is actually useful). At the same time, you're subjecting yourself to getting smacked around by Electric moves (not too relevant), Ice types (Hello Mamoswine), and hard body rock types (the always mentioned Mega Aerodactyl). On paper, 2 immunities and a resist is great but in practice not so much. What even uses Ghost type moves? Off the top of my head you have Chandelure and thats about it, not much of a consolation as Fire Blast damn near OHKO's you.
252+ SpA Chandelure Fire Blast vs. 4 HP / 0 SpD Mega Pidgeot: 258-304 (83.7 - 98.7%) -- guaranteed 2HKO
Grass types of the tier include Chesnaught, Roserade, and Tangrowth. Two of these sleep you as you come in, and one of them does a fuck ton with Sludge Bomb (Hint: It's Roserade). Now the Ground immunity is actually really nice, as it is with all Flying-types. However, when you look at it, the ground types of the tier still threaten Mega Pidgeot and it really doesn't want to come in. Swampert threatens Scald burn while dealing 30%, nothing to scoff at. In addition, Swampert could very well mega evolve itself and fucking demolish Pidgeot with a strong attack. Krookodile deals over half with Dread Plate Knock Off while not getting killed by Hurricane even after rocks. Finally, Gligar at the very least guarantees momentum for your team by chewing a Hurricane before U-turning out (Substitute variants are a nightmare for Gligar). So all in all here, middling bulk + garbage typing leave a lot to be desired.
Now, this third point also ties in to the previous ones. Pidgeot can't Mega evolve safely. If you've played with Pidgeot you know exactly what I'm talking about. Unless you're against some slow ass team then it is a pain in the ass to mega evolve. This is obviously due to the shitty bulk and typing that I just talked about. This point holds especially true versus offensive teams. Offense can pretty much OHKO Pidgeot whenever it gets to fire off a hit. This is true so much so that especially on the ladder, Pidgeot users attempt to evolve turn 1 against the lead (smart). However, simple tweaks make Pidgeot utterly useless by OHKOing it with TBolt from Azelf. From there it Hurricanes against something and then dies because it isn't strong enough to OHKO that many things.
These arguments thus far still fail to mention any of the counters in the tier such as Ampharos, Aerodactyl, Empoleon, Porygon2, Blissey (some sets), Florges (kinda), among others. Furthermore, you're still taking up your mega slot and all these together make for something that isn't quite S Rank material. For anybody who is having trouble against Mega Pidgeot I would suggest to play proactively and not re-actively. No shit you're going to get beat by it if you try to switch in on Hurricanes every time. Force that motherfucker to roost instead of hitting you and your problems are solved. Either that or bring an Ampharos and laugh as you get free momentum every time it comes in.
edit: holy fuck thsi got really long really fast
Pidgeot Stays A+ or wherever it is I didn't check.