(Sorry for any grammar mistake)
Well after thinking a bit...
Piplup is one of, if not, Dawn's strongest Pokemon despite that he gets constantly overshadowed by other Dawn's Pokemon during DP series (Buizel, Togekiss and especially Mamoswine)
He came up as a cocky but not-that-strong Pokemon at the start of DP series (He was easily beaten by a wild Buneary, Nando's Budew and Dawn's Buneary prior being caught (though he ends technically beating her at the end of the episode)) but after Dawn's first contest, Piplup growth very well in terms of battle, maybe due the implement of contest moves in his fighting style instead of just keep fighting as a regular Pokemon fit more to him, this was actually showed during his fight against Ash's Buizel prior being caught by Dawn, when he tried to fight normally, he was overpowered rather easily but after using contest oriented moves he was able to deal enough damage over Buizel to allow Dawn to catch him (impressive given that in the very same episode Buizel could endure a point-blank Iron Tail and a Volt Tackle from Ash's Pikachu).
As long as the series go, Piplup gets more strong despite being used mostly as a comic relief, he has showed to be consistently at least equal with Ash's Chimchar prior the halfway of the series (He beat it in a 1 vs 1 and tied with him in tag battle), give a good fight against Mayline's Lucario despite ending beaten, he kinda easily beat Lyra's Chikorita who was able to stand against Ash's Monferno and Dawn's Cyndaquil in tag battle (and I recall Piplup breaking through Chikorita's Light Screen, feat that not even Monferno Flamethrower + Cyndaquil's Swift could replicate), during Arceus Movie, he was able of overpower the Fire Spin of a Heatran and knock him down from a roof (given that this Heatran didn't seemed to feel a thing when he got hit by Pikachu's thunderbolt, this is kinda impressive) and by BW series Piplup was able to easily counter all of Cilan's Pansage attacks.
Piplup has also one of the most interesting combinations within a move in the series, and that's his capability of combining his Spin Dodge with his Bide attack, unfortunately it was only used back then in the Wallace Cup, Piplup would use the Spin Dodge to avoid being hit by the attack and then he would use the Bide to melt and absorb the energy of the attack to fired it back at the double of the potency, this is an amazing way to use bide given that Piplup doesn't get hit by the attack yet he still absorbs it's energy to fire it back against its foe.
Buneary by the other side doesn't have a lot of fights compared to Piplup nor she has a lot of hype/size as Mamoswine or Togekiss, but she isn't that weak. Despite that most of people complain about how Buneary was just captured to give Dawn a Pokemon that can use Ice Beam (which is kinda true), her freezing capabilities are amazing (she
easily created an Iceberg that oversized a ship with a single shot,
easily freeze one of the warp space things that were erasing Solaceon Town (and given that
it was needed the contribution of a bunch of trained and wild Pokemon to merely hold back this warp, this is kinda impressive) and she was able to
cold a rocket during a atmospheric re-entry from inside-out), and she is, despite her appearance, physically strong (she was able to stop Zoey's Glameow's Iron Tail with her bare ears (take in mind that in the very same episode Glameow's Iron Tail overpowered Aipom's focus punch) and one-shot a Phione, a 4 Gen Mythical/Legendary Pokemon, with a single kick). While I can agree that Buneary isn't Dawn's strongest Pokemon, she isn't weak at all compared to the average level.
As for Pachirisu, while he is clearly indeed to be Dawn's most cute Pokemon, he doesn't really seems to lack of strength nor raw power at all, he was able to, even if it wasn't a real fight, K.O most of Dawn and Ash Pokemon with a single attack early in the series, he beat Gardenia's Turtwig with a single Spark attack (Not even Ash's Staravia and Turtwig could do that) and he was able to counter Ursula's Gabite's attacks, even the
physical attacks from the same Gabite that
overpowered a Hariyama who according S/M series Pokedex, are able to send flying a 10-ton truck. He is also an incredibly speedy Pokemon.
I don't want to talk about Mamoswine. No particular reason behind of it, I just don't want.
Quilava it's definitely one of Dawn's strongest Pokemon, even as a baby Cyndaquil he was able to put a great fight against Ash's Grotle who previously put a great fight against Palmer's Rhypherior and actually keep himself in a fight against Lyra's Chikorita and Khoury's Totodile/Croconaw along with Ash's Monferno. He is actually Dawn's physical strongest Pokemon maybe aside from Mamoswine + Ice Sharp (You know, Mamoswine eating his own Ice Sharp to increase his power) given that he was able to break free out of the Psychic grip of that Shiny Wild Ariados who overpowered Dawn's team. Unfortunately, he lacks of fights.
There's no much I can say about Togekiss, I mean, she clearly isn't a weak Pokemon, but she is kinda Featless.
The only thing I can recall about Buizel while being in Dawn's hands aside from his capture episode was that Aqua Jet + Spin which was used by Ash to overpower Mayline's Lucario's Attacks.
Don't know what to tell about her Ambipom.
I would like to point that, in some places, a have seen people who talk about Dawn's Pokemon being weak because her Pokemon don't end their fights by knocking their foe, but I think that is mostly because Dawn doesn't really want to actually K.O her opponent's Pokemon, this was actually showed during Lilypad Contest when she gets upset when Mamoswine beat a fodder Granbull with a single hit.
Well... I think that this was a kinda pointless post, because I'm not answering the main question...
Dawn strongest Pokemon was that Grimer during the Summer Camp.
