What do you like most about Applejack?

[FallCake 19]

She embodies the merits of a hard day's work and is always willing to help out a friend.

[Astralshy 28,746]

Her honesty - I think this is not enough

She's grounded to the earth, keep a cool head in tough situations (most of the time). I also like her family spirit - even if it gets too much at times.

[lambert 7]

Her episodes were more family oriented and so liked that a lot about her! She's a good portrayal of typical country character accents in Media.

[Night Sky 628]

As so many other ponies have said, her honesty, 

Tell the truth and your problems will clear up.

As so many other ponies have also said, AJ is probably the most correctly balanced of the Mane 6.

Overlooking just a hint of periodic stubbornness, Applejack's down to earn mentality and penchant for plain English can clear up much confusion and tactfully address many elephants in the room.  

[cory377 1]

I really like how honest and down-to-earth she is. She’s always there for her friends and works super hard without bragging about it.