Mitch (M.A.) Larson
Even with fewer episodes than some, dude's written some epic episodes: The episodes with the 2 meaningful occurrences of the Sonic Rainboom, the Discord episodes, the first real appearance of Princess Luna, and ambiguously addressing Rarity's knowledge Spike's crush.
In the two Sonic Rainboom episodes (Sonic Rainboom and The Cutie Mark Chronicles,) Dash is a bit more lovable of a character (you know, like, instead of 3, it'd be, like, a 5 or something.) She actually seemed a little embarrassed about Scootaloo's admiration for her. Also, she was pretty upfront with her emotions in Sonic Rainboom: Ecstasy and anxiety and false bravado (and you had a couple of moments to see through that anyway.)
In Swarm of the Century, she's pretty much exactly as I would expect her to be (also points for continuity in language: "Pinkie Pie, you are so random.") In the Discord episodes, she's pretty bland, I think. Well, she was gone for a large portion of Part 2, but she was just basic Rainbow Dash. She flies around constantly and shoots her mouth off. I'm also pretty sure that Dash has the foulest mouth in Ponyville.
In Luna Eclipsed, she's kinda like A Bird in the Hoof Dash. But a little less chaotic. She didn't make much of an appearance in Spike's most recent episode.
Dave Polsky
wrote for South Park apparently. Explains everything, no? Anywho, Dash wasn't in Feeling Pinkie Keen, but she was more interesting than what I would expect from her. She's loyal to what she believes, she misses some things (asking too much of Spike when he already made plain that he worked plenty for them,) and she's very animated and aggressive.
Dave's episodes feel a little out of place, even though I thoroughly enjoyed both of them. Even though I like his writing for Rainbow Dash, I can't say that I think it's actually "Dash." But I'm sure Lauren had final say in that episode, so I guess it was close enough.
Merriweather Williams
I'll give Dash a pass for these episodes. It was crucial for Dash to be painfully arrogant for The Mysterious Mare Do Well, so she had to be Arrogant Dash for the episode to work. Between this and May the Best Pet Win!, my annoyance with Dash became dislike, but I think she was written accurately given the plot.
I give a pass for Hearth's Warming Eve, because of the two approaches I have to assessing the characters' portrayal in it.
1. The characters are acting. They are acting like they are acting, so the writers are writing for the characters as actors instead of charact…
You know what? F*ck it.
2. These are not the characters. Simply, Dash wasn't in the episode much. The leader of the Pegasi is different from Dash, so I have no idea how it should be written or if I like a character that isn't Dash as Dash.
I'd also like to point out that, like most any Pony discussion in this thread, a lot of this is baseless rambling. The show presents Dash to be a fairly consistent character, even when the plot designates her to be something specifically.
For example, I said Dash has the fowlest mouth in Ponyville, yes? (It's in there. Somewhere.)
She says "Horseapples" in Fall Weather Friends, "What the hay" in Applebuck Season, and "I got a message for you too" (>implying that it wasn't anything nice) in the Part 1 of Return of Harmony.
But those were written by two different writers. And I probably missed a couple of other euphemistic instances of profanity from Rainbow Dash in other episodes. Perhaps the writing isn't as cut and dry as "The writer who appears on the credits is the only writer for that episode," and maybe Lauren communicated to the writers directly or indirectly that Dash is pretty much a potty mouth (given that this is Equestria.)
tl;dr: M.A Larson writes the Best Rainbow Dash.