Applejack is the opposite of sloth, she’s a workaholic.

AJ: Apples done…nothing more to do here.
PP: Must get all the pastries!!
FS: Life is so unfair. I wish ponies could stop mock me.
S: Hehehe, I love this gems, good stuff indeed.
RD: I should be the wonderbolt leader, I make a better job.
RR: Ohh darling, just give into temptation, I know you want it.
TS: Is hard to be smart when you are surrounded by fools.

Twilight Sparkle-Pride the Arrogant  
Rarity-Lust the Lascivious  
Pinkie Pie-Gluttony the Voracious  
Rainbow Dash-Envy the Jealous  
Applejack-Sloth the Indolent  
Spike-Greed the Avaricious  
Fluttershy-Wrath the Furious

@Background Pony #6DF5  
I honestly don’t see Twilight as envy but I can totally see Rainbow as pride.

@Background Pony #E917  
Twilight could fit for Envy  
Rainbow ? Not so much

@Background Pony #E917  
Yeah. Twi is the odd one out a bit, but really most of them are twisted versions of themselves in this thematic piece. They’ve all shown traits to suggest they would fit, if they were warped enough.
 
Twilight - Take away her anxiety and the whole friendship focus she has, and she’d probably be pretty prideful. Princess Celestia’s prize pupil, etc. She was shown to have some degree of pride, she was just more worried about her friends and teacher thinking poorly of her for (reasonable) pride.
 
RD - Yeah, more of a pride thing. But Envy does tie into it.
 
Pinkie - Yeah, Gluttony fits her. She somehow manages to not be fat somehow.
 
Rarity - She is a bit of a flirt when it suits her, and certainly threw herself at her ‘prince’ way back when. She also gets rather excited about fancy things. Turn it all up to 11 and you get Lust.
 
Fluttershy - Flutterage is a well known thing, just leave it on all the time and make the target ‘anyone that annoys her’.
 
Applejack - Doesn’t super suit her, but she does (naturally and deservedly) rest when work is done. Just make her stop caring about her sense of duty and she’d probably be pretty lazy.
 
Spike - Yeah, we’ve seen his greed getting out of control.

Rainbow Dash should totally be pride  
Twilight doesn’t fit anything because she’s a sue

Spike = Greed  
Applejack = Sloth  
Rarity = Lust  
Fluttershy = Wrath  
Rainbow Dash = Envy  
Pinkie Pie = Gluttony  
Twilight Sparkle = Pride

Twilight Sparkle-Pride  
Rainbow Dash-Envy  
Pinkie Pie-Gluttony  
Applejack-Sloth  
Rarity-Lust  
Fluttershy-Wrath  
Spike-Greed

Envy, pride, lust, glutonny