Ah, the humble Pidgeot. For many of us, Pidgey was the first Pokemon we ever caught (either him or Rattata). Personally, I always preferred Fearow because Drill Peck was THE best Flying move in Gen 1, but Pidgeot is still a classic Pokemon that I remember fondly. It also started the trend of where you can always find at least one small bird Pokemon early in your Pokemon adventure. And as time went on, Pidgeot became more and more overshadowed by the evolved forms of the bird Pokemon in later Gens. Noctowl and Unfezent aren't too impressive, but Swellow, Staraptor, and especially Talonflame all left Pidgeot in the dust. The only thing Pidgeot really had to stand out was its decent defenses, and even then there are much better defensive Flying-types out there, like Skarmory.
But then ORAS came around and finally gave Pidgeot a niche with its new Mega Evolution. Mega Pidgeot does one thing really, really well: it can spam Hurricane like no other. Thanks to No Guard, there's no need for rain or for crossing your fingers about that shaky 70% accuracy. Mega Pidgeot's highly boosted Speed and Sp. Attack also really help it in this regard, and it still keeps its decent defenses so it won't drop in one hit. Sadly, the good news ends there as Pidgeot's special movepool is about as shallow as a puddle on a hot summer day, and it has no means of boosting its stats. That makes it an extremely predictable Pokemon. Even then, most Pokemon are not going to like taking a STAB Base 120 power move off a base a Base 135 Sp. Attack with no drawbacks.
Recommended tier: RU
Big Pecks keeps your Defense stat from lowering. This generally means you just have insurance against Defense drops from Crunch. Very situational.
Keen Eye keeps your accuracy from lowering and ignores the foe's Evasion boosts. As Evasion-raising moves are banned and the only commonly used Accuracy-lowering move is Muddy Water, this is also very situational.
Tangled Feet raises your Evasion one level when you're confused. Confusion is a rare status, but of the three this is probably the one you'll come across the most, making this arguably the best. Still not great, though.
Mega Evolution: No Guard is what makes Mega Pidgeot so good at what it does. It ignores all rules of accuracy and makes all of your moves and your opponents moves always hit. This means Mega Pidgeot can churn out Hurricanes and Heat Waves like its nobody's business, although it also must be very wary of opponents carrying Stone Edge (or less commonly, Thunder or Blizzard).
Hurricane and Heat Wave are Mega Pidgeot's two key moves. Hurricane is of course, your main STAB, while Heat Wave also benefits from No Guard and covers Steel-types. Aside from these two, literally the only special attack worth running on Mega Pidgeot is Hidden Power. The type of it depends on your needs. Smogon recommends HP Grass because of Rhyperior. U-Turn is also good for Mega Pidgeot because it can escape a counter and keep offensive momentum. Roost gels well with Mega Pidgeot's decent defenses, and its high Speed means it will usually go first and remove its Flying-type for the turn. Tailwind is good for a Doubles Mega Pidgeot, as it is one of the faster Tailwind users. And that's...pretty much it. Everything else it packs is hindered by low base power or poor coverage. Like I said, a predictable Pokemon, but not a bad one.
Countering Pidgeot:
You can generally expect to see Mega Pidgeot when your opponent is running Pidgeot, and the best way to defeat it is to use its No Guard ability against it. Bring out those Stone Edges! Rhyperior counters Mega Pidgeot without HP Grass, but is 2HKO'd otherwise. Mega Pidgeot may U-Turn out of your counter, though, so be prepared to switch as necessary. Because Mega Pidgeot is so fast, revenge killing can be tricky, but Choice Scarfed Pokemon are your best bet. Also, its defenses are decent but far from the best, and its weak to Stealth Rock, meaning it will still fall to a strong enough attack.