Which Actress Under 35 has the best filmography?

Keira Knightley, probably.  If you bump it to under 36, it's Scarlett Johansson.  

Okay, just had a look at the filmographies of the Fanning Sisters....and I may have to change my mind again. I think it's probably Elle. Though Dakota's is pretty solid as well.

Both have done a lot since they were children and covered every concievable genre.

Ronan, Stone, Elle Fanning.

Possibly also Adèle Haenel, but I'm not sufficiently acquainted with her filmography to say for sure.

Rooney Mara

Gotta give it up to Emma Stone. Birdman and The Favourite are all-timers for me, and La La Land is damned close. Ronan's a worthy second-placer, but she doesn't have any films in her catalogue on the level of those peaks yet.

Knightley and Rooney Mara are pretty strong. Saoirse Ronan is off to a really good start.

Hard to vote against Stone, she's doing very well. Less obvious actresses like Carey Mulligan Anna Kendrick, Evan Rachel Wood have interesting and somewhat underrated filmographies imo.

And Yeah, even if Man On Fire is the only one film really worth mentioning, what Dakota Fanning was able to do as the Hollywood’s go-to young girl; almost/pretty much co-starring with Denzel, De Niro, Cruise and Penn in 4 years at 11 is unique.

True about Dakota but why hasnt her career gotten back on track?

She just had a notable role in Once Upon A Time In Hollywood, so things could be much worse. Outside of this year's arguable Best Picture frontrunner, she still gets cast in notable projects, that for one reason or another dont end up working or catching fire ( like Ewan McGregor's American Pastoral) but she is constantly working.

I think the industry still believes in her. She's just going through that in-between phase where she's no longer the cute kid, but still looks too young (even though she's in her mid 20's now) to be the most obvious casting as grown women. A lot of her competition (Jennifer Lawrence, Emma Stone etc) read as more mature physically, so get the more can't miss projects. Sairose Ronan takes all the best prestige roles that might suit her current playing age, and you always have another "it girl"  that breaks out every year who makes it even harder (ie Florence Pugh).  Not to mention her own sister. She's someone who is in the position to pull a Jodie Foster, when she starts looking the part. Her career is still relatively good and she seems well liked in the industry, so I can see her becoming a big thing again at some point.