I'd like to nom Mega Swampert for
A- or
A rank. Reason being that Mega Swampert is far and away the best Swift Swim user atm and is a relevant powerhouse outside of rain. I'd also argue he is the best mega pokemon to use within the context of a rain team. I'll just go down the relevant threats from S down
S:
252+ Atk Mega Swampert Ice Punch vs. 64 HP / 0 Def Mega Altaria: 184-218 (59.9 - 71%) -- guaranteed 2HKO
252+ Atk Mega Swampert Waterfall vs. 252 HP / 252+ Def Clefable in Rain: 211-249 (53.5 - 63.1%) -- guaranteed 2HKO after Leftovers recovery
A+:
252+ Atk Mega Swampert Earthquake vs. 88 HP / 4 Def Azumarill: 238-282 (65.5 - 77.6%) -- guaranteed 2HKO
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 252+ Def Garchomp: 300-356 (71.4 - 84.7%) -- guaranteed 2HKO
252+ Atk Mega Swampert Low Kick (120 BP) vs. 0 HP / 4 Def Mega Gyarados: 296-350 (89.4 - 105.7%) -- 37.5% chance to OHKO
252+ Atk Mega Swampert Earthquake vs. 0 HP / 0 Def Keldeo: 219-258 (67.8 - 79.8%) -- guaranteed 2HKO
252+ Atk Mega Swampert Ice Punch vs. 0 HP / 0 Def Latios: 240-284 (80.2 - 94.9%) -- guaranteed 2HKO
252+ Atk Mega Swampert Earthquake vs. 96 HP / 0 Def Manaphy: 199-235 (54.5 - 64.3%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 252 HP / 252+ Def Mega Sableye in Rain: 148-175 (48.6 - 57.5%) -- 94.9% chance to 2HKO
252+ Atk Mega Swampert Waterfall vs. 248 HP / 0+ Def Mega Scizor in Rain: 163-193 (47.5 - 56.2%) -- 84% chance to 2HKO
252+ Atk Mega Swampert Waterfall vs. 132 HP / 0 Def Tornadus-T in Rain: 289-342 (87 - 103%) -- 18.8% chance to OHKO
A:
252+ Atk Mega Swampert Low Kick (100 BP) vs. 252 HP / 88+ Def Ferrothorn: 180-212 (51.1 - 60.2%) -- 89.1% chance to 2HKO after Leftovers recovery
252+ Atk Mega Swampert Low Kick (120 BP) vs. 56 HP / 0- Def Kyurem-B: 356-420 (87.9 - 103.7%) -- 25% chance to OHKO
252+ Atk Mega Swampert Ice Punch vs. 72 HP / 0 Def Latias: 218-258 (68.3 - 80.8%) -- guaranteed 2HKO
252+ Atk Mega Swampert Waterfall vs. 248 HP / 216+ Def Rotom-W in Rain: 85-101 (28 - 33.3%) -- 94.3% chance to 4HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 252 HP / 252+ Def Skarmory in Rain: 136-162 (40.7 - 48.5%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Mega Swampert Earthquake vs. 252 HP / 232+ Def Slowbro: 136-162 (34.5 - 41.1%) -- 64.6% chance to 3HKO after Leftovers recovery
252+ Atk Mega Swampert Earthquake vs. 248 HP / 0 Def Starmie: 229-270 (70.8 - 83.5%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Mega Swampert Earthquake vs. 252 HP / 240+ Def Mega Venusaur: 126-148 (34.6 - 40.6%) -- guaranteed 3HKO
A-:
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 148+ Def Celebi: 158-186 (39.1 - 46%) -- guaranteed 3HKO
-1 252+ Atk Mega Swampert Waterfall vs. 88 HP / 4 Def Gyarados in Rain: 97-114 (27.4 - 32.2%) -- 60.9% chance to 4HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 252 HP / 104+ Def Mew in Rain: 198-234 (49 - 57.9%) -- 56.3% chance to 2HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 0 HP / 0 Def Mega Pinsir in Rain: 205-243 (75.6 - 89.6%) -- guaranteed 2HKO
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 0 Def Serperior: 210-248 (59.3 - 70%) -- guaranteed 2HKO
252+ Atk Mega Swampert Earthquake vs. 252 HP / 252+ Def Mega Slowbro: 94-112 (23.8 - 28.4%) -- 96.7% chance to 4HKO
B+:
252+ Atk Mega Swampert Ice Punch vs. 0 HP / 4 Def Multiscale Dragonite: 208-246 (64.3 - 76.1%) -- guaranteed 2HKO
252+ Atk Mega Swampert Waterfall vs. 0 HP / 4 Def Mega Heracross in Rain: 213-252 (70.7 - 83.7%) -- guaranteed 2HKO
252+ Atk Mega Swampert Waterfall vs. 192 HP / 64 Def Reuniclus in Rain: 280-330 (68.4 - 80.6%) -- guaranteed 2HKO
252+ Atk Mega Swampert Earthquake vs. 248 HP / 0 Def Slowking: 240-283 (61 - 72%) -- guaranteed 2HKO
252+ Atk Mega Swampert Earthquake vs. 252 HP / 252+ Def Suicune: 130-154 (32.1 - 38.1%) -- 0.6% chance to 3HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 248 HP / 0 Def Togekiss in Rain: 252-297 (67.5 - 79.6%) -- guaranteed 2HKO after Leftovers recovery
B:
252+ Atk Mega Swampert Superpower vs. 4 HP / 252+ Def Eviolite Chansey: 424-500 (66 - 77.8%) -- guaranteed 2HKO
252+ Atk Mega Swampert Earthquake vs. 0 HP / 4 Def Feraligatr: 199-235 (63.9 - 75.5%) -- guaranteed 2HKO
252+ Atk Mega Swampert Ice Punch vs. 0 HP / 0 Def Mega Garchomp: 356-420 (99.7 - 117.6%) -- 93.8% chance to OHKO
252+ Atk Mega Swampert Waterfall vs. 252 HP / 252+ Def Quagsire in Rain: 192-226 (48.7 - 57.3%) -- 47.7% chance to 2HKO after Leftovers recovery
252+ Atk Mega Swampert Earthquake vs. 224 HP / 252+ Def Tentacruel: 374-444 (104.7 - 124.3%) -- guaranteed OHKO
252+ Atk Mega Swampert Waterfall vs. 252 HP / 168+ Def Zapdos in Rain: 210-247 (54.6 - 64.3%) -- guaranteed 2HKO after Leftovers recovery
B-:
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 136+ Def Amoonguss: 204-242 (47.2 - 56%) -- 21.5% chance to 2HKO after Black Sludge recovery
252+ Atk Mega Swampert Waterfall vs. 252 HP / 0 Def Bronzong in Rain: 211-249 (62.4 - 73.6%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 252+ Def Chesnaught: 126-150 (33.1 - 39.4%) -- 19.3% chance to 3HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 0 HP / 0 Def Conkeldurr in Rain: 252-297 (71.7 - 84.6%) -- guaranteed 2HKO
252+ Atk Mega Swampert Waterfall vs. 248 HP / 136+ Def Mandibuzz in Rain: 184-217 (43.4 - 51.3%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 252 HP / 220+ Def Sylveon in Rain: 234-276 (59.3 - 70%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 4 Def Tangrowth: 166-196 (41 - 48.5%) -- guaranteed 3HKO
C+ or lower:
252+ Atk Mega Swampert Earthquake vs. 120 HP / 136 Def Alomomola: 204-241 (40.7 - 48.1%) -- guaranteed 3HKO after Leftovers recovery
252+ Atk Mega Swampert Waterfall vs. 52 HP / 232+ Def Wobbuffet in Rain: 246-289 (46 - 54.1%) -- 3.9% chance to 2HKO after Leftovers recovery
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 252+ Def Gourgeist-Super: 126-150 (33.6 - 40.1%) -- 35% chance to 3HKO after Leftovers recovery
252+ Atk Mega Swampert Ice Punch vs. 252 HP / 252+ Def Whimsicott: 160-190 (49.3 - 58.6%) -- 98.8% chance to 2HKO
252+ Atk Mega Swampert Earthquake vs. 252 HP / 16+ Def Filter Mega Aggron: 127-153 (36.9 - 44.4%) -- guaranteed 3HKO
Anything I didn't mention is
OHKO'd by a move of the standard set (Waterfall, Earthquake, Ice Punch, Low Kick/Superpower) or it doesn't have any offensive pressure relevant to counter Swampert's 2HKO.
Notice in most of the calcs I chose some of the bulkiest sets those mons could run against Swampert, albeit some like the Ferrothorn calc I took into account his most popular set rather than the bulkiest.
Even in Swamperts worst matchups the damage he brings is fairly significant (around 30% damage to bulky Gyarados after Intimidate and about the same to Def Rotom-W)
Swampert's bulk is the next consideration and I won't go into as much detail as I did with how much damage he can bring here's some of his more impressive feats:
252 SpA Life Orb Latios Draco Meteor vs. 100 HP / 0 SpD Mega Swampert: 255-302 (69.6 - 82.5%) -- guaranteed 2HKO
252+ Atk Choice Band Talonflame Brave Bird vs. 100 HP / 0 Def Mega Swampert: 217-256 (59.2 - 69.9%) -- guaranteed 2HKO
252 SpA Keldeo Hydro Pump vs. 100 HP / 0 SpD Mega Swampert in Rain: 247-292 (67.4 - 79.7%) -- guaranteed 2HKO
252 Atk Tough Claws Mega Metagross Zen Headbutt vs. 100 HP / 0 Def Mega Swampert: 174-205 (47.5 - 56%) -- 80.9% chance to 2HKO
+1 192+ Atk Pixilate Mega Altaria Return vs. 100 HP / 0 Def Mega Swampert: 280-330 (76.5 - 90.1%) -- guaranteed 2HKO
252 Atk Garchomp Outrage vs. 100 HP / 0 Def Mega Swampert: 181-214 (49.4 - 58.4%) -- 98.8% chance to 2HKO
252+ Atk Choice Band Terrakion Close Combat vs. 100 HP / 0 Def Mega Swampert: 297-349 (81.1 - 95.3%) -- guaranteed 2HKO
+2 84 Atk Mega Scizor Knock Off vs. 100 HP / 0 Def Mega Swampert: 130-154 (35.5 - 42%) -- guaranteed 3HKO
Basically in a pinch Swampert can tank pretty much any of the heaviest hitters and guarentee he wins with rain speed. Obviously his typing is excellent with only one weakness and key resists to Steel, Rock, and Fire. He takes a resisted Stealth Rock which in many cases means he won't be surprise OHKO'd by any of the ones listed there that get up into the 90% range. Most importantly he has an electric immunity allowing him to switch into mons like non Grass Knot Thundurus-I and Raikou giving him a free mega evolve and making him the only swift swimmer besides Seismitoad (lol) to be unaffected by Thunder Wave.
A core of Politoed, Torn-T, and MSwamp is enough to climb 500 points on the ladder on its own atm and that's why he deserves at least
A- rank if not
A because personally I see him as more powerful than all the offensive threats in
A-(MAero, MPinsir, Alakazam, Serperior, Volcarona). MSwamp is easier to set up than Serperior, MPinsir, and Volcarona and it is bulkier and less susceptible to counters than MAero and Alakazam. It's main flaw is it is less "plug-and-play"-able then those mons but it can setup its own rain with the cost of either Dragon or Dark/Steel coverage. It seems just as functional in my eyes that way as Volcarona who would have less Speed and potentially less fire power if Swampert can spam Waterfall.