@RektPony  
Fair enough, but I don’t think tose rankings are necessaril set in stone. (What? Too soon?)
 
And I stand by my theory on why Discord doesn’t seem to like Pinkie despite their seeming similarities. He prefers that he be the center of attention.

@Scrounge  
It’s from Discord point of view. When he needed the ticket for the gala he first came to Twilight as his second best. Then he tried with the rest of mane6. Pinkie was least favourite and he become desperate to ask her. However he actually gave up and went back to home rather than asking Celestia for tickets. Then he also ignored her at the gala even though he got them eventually. Celestia might like him or at least his chaos from time to time but for now it is clear that Discord do not like her more than Rainbow Dash.  
Spike is a different story. Both his interaction in Twilight Kingdom and Smooze episode had Discord bullying him. So he is probably beneath Celestia. He also said “And her little dragon too” so it kind of sounds like he treat him as a pet.

@RektPony  
I dunno, Celestia actually laughs at his jokes. Twilight barely puts up with him and he just hangs around her because she’s fun to annoy.
 
And as for your list, I presume Spike doesn’t count and Discord considers him basically Twilight’s pet?

@Scrounge  
Celestia is probably on the last place when it comes to befriending from his point of view. It goes like this  
Fluttershy>Twilight>Smooze>Pinkie>Rarity>Applejack>Rainbow Dash>Luna>Celestia

@RektPony  
Yeah, out of the Mane Six I think it’d likely be either Pinkie Pie or Rarity that’d form a bond with him next, if any of them do. And for some reason I don’t think he cares much for Pinkie because he can’t handle not being the weirdest person in the room. But Rarity is an artist, or at the very least considers herself one, and I suspect that Discord might be able to help her see the world differently when her creative juices are running low (even if he might be reluctant to bring her back to seeing the world normally afterwards).
 
Although I think his second best friend after Fluttershy is, at this point, either Celestia or the Smooze.

Well, AJ, you reeeeeally messed up.

@Alara Rogers  
Exactly. While I can’t really see them as a ship in any way (though I don’t mind those who do), its precisely the fact that they’re so different which interests me so much. I really think they can pull off an episode of bonding between then two, just by virtue of how polar opposites they are, and how AJ would really be the last of the Mane6 anyone would expect to open up to him.

@RektPony  
I could see them eventually becoming friends of a sort; they have almost nothing in common, except their mutual friends, but that in itself is a bond that could develop. I really can’t see shipping, though. I have a few friends who swear by that ship, but… I can see literally any other Discord ship more easily than that one.

I don’t really see AJ and Discord ever being in good terms with eachother. Even his second best friend after Fluttershy, Twilight seem to dislike him. And she can at least admire his powerfull magic and spells. Applejack isn’t interested in both the magic nor jokes and isn’t patient enough to bear Discord.

@Maya  
And it desperately needs to be.  
As the most down to earth and level-headed member of the Mane6, seeing AJ interacting with Discord is pure gold.

“AJxDiscord”
 
not in this world