step up, snap ya back
In spite of its odd stat spread, or rather because of it, Pelipper is actually pretty good in NU. An immunity to Spikes and an ability to counter Hitmonchan are both amazing qualities for a defensive Pokemon, and a neutrality to Grass really separates it from the pack of other bulky Water-types. Pelipper also sports a decent movepool and can go defensive or offensive, allowing for some flexibility when considering it for a teamslot. Pelipper's horrid weakness to Electric-type attacks means it's pretty easily revenge killed by fast special attackers, and its weakness to Rock-type attacks means that despite being a Water-type it cannot safely check Rock-types like Pupitar, but those shortcomings are eclipsed by Pelipper's positives.
name: Physically Defensive
move 1: Surf
move 2: Toxic
move 3: Ice Beam / Rest
move 4: Protect / Sleep Talk
ability: Keen Eye
evs: 252 HP / 252 Def / 4 Spe
Thanks to its good typing and an immunity to Spikes, Pelipper makes one of the best defensive switch-ins to a multitude of dangerous Choice Band users, including the highly feared Hitmonchan (if it lacks the uncommon Rock Slide) and others such as Metang, Glalie, and Whiscash. Thanks to its immunity to Earthquake and access to STAB Surf, Pelipper also makes a decent check to Rock-types like Sudowoodo and Graveler despite being unable to take repeated hits from Rock Slide. Surf and Ice Beam allow Pelipper to have a decent offensive presence on the field, Ice Beam in particular making Pelipper a decent check to Grass-types, something its competition such as Wailord has some trouble doing. Toxic whittles down those that fear neither attacking move, particularly opposing Water-types. Protect pairs perfectly with Toxic and allows Pelipper to scout the opponent's Choice Band moves and stall for Leftovers recovery. If you desire Pelipper to have greater longevity, RestTalk is usable in the last two slots and can allow Pelipper to counter Hitmonchan throughout the match.
Hidden Power Grass is a decent choice to consider on this set as Pelipper will usually beat other Water-types one-on-one thanks to its own neutrality to Grass. In particular, Hidden Power Grass makes Pelipper one of the best checks to Whiscash, Seadra, and Relicanth in the tier. Hidden Power Flying can be used over Surf in order to 2HKO Choice Band Hitmonchan whereas Surf is only a 3HKO, but Surf leaves Pelipper less vulnerable to a larger variety of threats. In the same vein, Hydro Pump is also an option as Pelipper's main STAB move, but the reliability of Surf is generally more desired on a defensive set such as this.
Fast special attackers are the biggest threat to Pelipper's success, especially those with Electric-type attacks, making a specially defensive partner that's able to absorb them a necessary partner. Specially defensive Bellossom and Roselia in particular form a potent core with Pelipper as they cover each other's weaknesses almost perfectly and can stall out many other defensive Pokemon between Leech Seed and Toxic. Metang and Mawile are excellent partners to consider to fill out this core as they cover the Normal-type Pokemon Pelipper can't handle such as Vigoroth and Pidgeot. As Pelipper covers Hitmonchan very well, Pokemon that are weak to it will appreciate having Pelipper watch their backs. Pokemon such as Raticate, Lairon, and Piloswine all love having Pelipper as a teammate.
name: Special Attacker
move 1: Hydro Pump
move 2: Ice Beam
move 3: Hidden Power Grass
move 4: Agility / Toxic
ability: Keen Eye
evs: 4 HP / 252 SpA / 252 Spe
Pelipper faces some harsh competition from Seadra and especially Huntail as a special attacker, but Pelipper's strength comes from its surprise factor and the ease of setting up thanks to its superior typing. The ability to set up on Hitmonchan can not be understated, and Pelipper's bulk and lack of Grass-type weakness means it has an easier time setting up on defensive threats like Sableye and opposing Water-types. With a Modest nature and an Agility boost, Pelipper outspeeds the whole tier up to Huntail in the rain, and has all the coverage it needs between Hydro Pump, Ice Beam, and Hidden Power Grass. Alternatively, if you would prefer to play Pelipper as more of a tank, Toxic can be used over Agility to cripple a wider variety of defensive threats such as Bellossom and Dewgong. Surf can be used over Hydro Pump if you prefer accuracy, although Pelipper really needs the power behind Hydro Pump to make it worth using over its competition.
A Timid nature can be used if you would prefer to Speed tie Adamant Hitmonchan and other maximum Speed base 65s like Flareon, but Pelipper will already outspeed Jolly Pupitar with a Modest nature before boosting, and outspeed almost everything it would want to after an Agility boost. A Timid nature does allow Pelipper to outspeed Tangela in the sun and Vigoroth after a Salac Berry boost, but those are the only relevant benchmarks over Modest. Conversely, only 44 Speed EVs can be run with the rest invested in HP to make the most of Pelipper's bulk. This will allow Pelipper to outspeed minimum Speed base 70s before boosting and +1 Jolly Pupitar and unboosted Raticate after using Agility.
This set fits best on offensive teams that need a strong switch-in to Hitmonchan but want to keep offensive pressure on the opponent. Pairing Pelipper with offensive partners that have good type synergy with it, such as Dragon Dance Pupitar and Agility Metang, is recommended. Spikes are also important to help weaken the switch-ins to Pelipper, and Roselia has great type synergy with Pelipper as well. Finally, Pokemon that can provide easier set up, such as Substitute Mawile, make great partners as well. Mawile in particular is a nice choice as it lures in the Ground-, Fighting-, and Fire-types that Pelipper has an easy time setting up on.
Blizzard is a good choice to consider as Pelipper has an easy time dealing with the Grass-types in NU, and the power difference between it and Ice Beam is pretty noticeable albeit at a severe accuracy cost. Icy Wind is another interesting option that can turn the tables on special attackers like Haunter that switch directly into Pelipper, only for them to be KOed by Icy Wind followed by Surf or Hydro Pump. Pelipper is one of the few Pokemon in NU with access to priority in Quick Attack, but Pelipper's Attack stat is pretty pitiful. Thief is a very cool option that can really screw with opposing bulky Water-types and other defensive Pokemon, but it is not a consistent option. Finally, Pelipper is one of the few Pokemon with access to Pursuit in the tier, and Pelipper actually has a decent Special Attack stat to make use of it. However, it won't do much to Pelipper's would-be main targets Hitmonchan and Flareon, and Pelipper isn't scaring out Chimecho or Haunter any time soon.
[Checks and Counters]
Chimecho, especially those with Hidden Power Electric, will probably have the easiest time consistently switching into Pelipper and forcing it out thanks to its good special bulk and the threat of using Pelipper as Calm Mind set up bait. Other special attackers such as Haunter, Plusle, and Pikachu have a harder time switching into Pelipper, but once in will easily OHKO Pelipper with Thunderbolt. Very strong Choice Band users such as Kingler and Pidgeot will still 2HKO even maximum Defense Pelipper with Double-Edge, but need to be wary of getting locked into a move that Pelipper is immune or resistant to. Finally, defensive RestTalk Pokemon such as Kecleon and Lickitung are probably the safest overall switch-ins to Pelipper, but they will at most stalemate with Pelipper if it is using RestTalk itself.