S Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame with little support, and Pokemon who can support other Pokemon with very little opportunity cost ("free turns"). Also the home of Pokemon who can easily perform multiple roles effectively, increasing their versatility and unpredictability. If the Pokemon in this rank have any flaws, those flaws are thoroughly mitigated by their substantial strengths.
Aegislash - Swords Dance sets are mildly threatening, but Amph is slower, so when Slash does choose to attack, Amph can almost always survive the hit, and OHKO with Thunderbolt
Bisharp - Not a problem. Even when uninvested in bulk, Bisharp's Sucker Punch does around 49% on average, whereas Focus Blast does around 350%. Even T-Bolt has a 94% chance to OHKO after Stealth Rock
Charizard (Mega-X) - Yeah, don't leave in Amph against this guy.
Charizard (Mega-Y) - Mega Amph wins 1v1 with Zard-Y every time. Y's earthquake does a measly 33.3 - 39.5%, whereas T-Bolt does 114.7 - 135.5%. Check there.
Deoxys-D - T-Bolt does about 32.8 - 38.8% against specially defensive Deo-D, so like with any other 'mon facing this brick wall, you can wear it down, but it'll take a bit.
Deoxys-S - T-Bolt does between 69.4 - 81.9%, so guaranteed 2HKO.
Landorus - You'd have to be stupid to leave in Amph here, but stunningly, if using the Specially Defensive set (which you should), 1v1, Ampharos WINS. The calcs to prove it:
252 SpA Life Orb Sheer Force Landorus Earth Power vs. 252 HP / 252+ SpD Mega Ampharos: 307-367 (79.9 - 95.5%) -- guaranteed 2HKO
4 SpA Mold Breaker Mega Ampharos Hidden Power Ice vs. 4 HP / 0 SpD Landorus: 324-384 (101.2 - 120%) -- guaranteed OHKO
Thundurus - Again, Amph wins 1v1 here as well. Amph's T-Bolt does around 77.3 - 91.3%, whereas Thundy's HP Ice does around 30.7 - 36.4%.
A Rank: Reserved for Pokemon who can sweep or wall significant portions of the metagame, but require some support or have some flaws that prevents them from doing this consistently. Supporting Pokemon in this rank may give opponents free turns or cannot create free turns easily themselves, but can still do their job most of the time
A+ Rank
Azumarill - Should you choose to run an Agility set, or just Max Speed + Jolly, or really just do something to outspeed Azumarill, you'll win here. If not, Amph will fall to the blue bunny's mighty Play Rough.
Excadrill - When invested in Defense, Excadrills EQ does around 67.1 - 79.6%, whereas Amph's Focus Blast does around 122.8 - 143.1%. The best idea is to nail him on the switch.
Garchomp - Lose
Greninja - If Specially Defensive w/ Focus Blast, win
Gyarados (Mega) - If unboosted, win
Keldeo - Win
Kyurem-B - Lose
Mawile (Mega) - Lose
Pinsir (Mega) - If unboosted, win, if boosted, lose to EQ
Scizor (Mega) - If unboosted, win. If SD is up, lose to Superpower
Talonflame - Hard counter to Talonflame, win
Tyranitar (Mega) - If unboosted, win with Focus Blast. If boosted, lose to EQ
Venusaur (Mega) - Stally as hell, just switch out
A Rank
Clefable - Assuming CM, lose. Switch out.
Dragonite - If multiscale is broken by SR, win. If not, lose
Ferrothorn - Win
Gengar - Win
Heatran - Win
Hippowdon - Lose
Landorus-T - Lose
Latios - Lose unless stat drop from Drago Meteor already occurred, at which point win. But basically lose
Manaphy - Win
Mandibuzz - Win
Rotom-W - Win
Terrakion - Win
Tyranitar - Win
A- Rank
Chansey - Neither, switch out
Garchomp (Mega) - Lose
Gyarados - Win
Latias - Lose
Mamoswine - If invested in defense, win with Focus Blast. If not, lose to EQ
Politoed - Win
Scizor - Win
Skarmory - OHKO thanks to Mold Breaker
Zapdos - Win
B Rank: Reserved for Pokemon who cannot sweep through or wall significant portions of the metagame, but can properly fulfill a given offensive/defensive niche. Support Pokemon in this category have flaws that prevent them from doing their job or are setup bait for dangerous sweepers. Pokemon who are partially outperformed by a Pokemon in A or S Rank, but are otherwise very dangerous, may also fall into this category.
B+ Rank
Breloom - Win
Conkeldurr - Lose
Diggersby - Lose
Gardevoir (Mega) - Lose
Gliscor - If nailed on the switch with HP Ice, win. If not, lose 1v1
Kabutops - Win
Manectric (Mega) - Win
Medicham (Mega) - Iffy, usually lose
Raikou - Win against non-assault vest variants
Scolipede - Win
Slowbro - Win
B Rank
Chesnaught - Win
Heracross (Mega) - Lose
Kingdra - If Specially Defensive, Win
Lucario - Win
Mew - Very much depends on the set
Quagsire - Win (EQ doesn't do much, and Quag has lower speed and shit SDef)
Suicune - Before the Calm Mind shit starts, win. Pray for a crit after.
Sylveon - Lose
Tornadus-T - Win
B- Rank
Absol (Mega) - Win
Alakazam - Win
Alakazam (Mega) - Win
Amoonguss - Depends on the set/status, usually win. Better to switch out.
Blastoise (Mega) - Win
Crawdaunt - Win
Gothitelle - Win
Rhyperior - If defensive, win. If not, lose
Staraptor - Win
Thundurus-T - Thanks to mold breaker, unboosted, win. After Nasty Plot, lose.
Victini - Win
Volcarona - Depends on the switches/Quiver Dances set up, but 1v1, win