I think these are the 16 female songs with 400k+ digital weeks, w/current iTunes position:
Hello 76
We Are Never Ever Getting Back Together 1477
Tik Tok
I Knew You Were Trouble 764
Roar 388
Shake it Off 196
Born this Way
Blank Space 355
Firework 443
Bad Romance
Wrecking Ball 909
Problem 897
Just Dance
Hold it Against Me
We R Who We R
Part of Me

I haven't heard the last one on the list in forever and don't remember the lyrics, so I choose that.

We are who we are or hold it against me

Hold It Against Me

We Are Never Ever Getting Back Together
I Knew You Were Trouble

probably Problem or Just Dance

We Are Never Ever Getting Back Together
I Knew You Were Trouble
Blank Space
Problem
Hold it Against Me
We R Who We R

Part of Me

Part of Me and Hold It Against Me

Just Dance
Hold it Against Me
We R Who We R
Part of Me
We Are Never Ever Getting Back Together
Wrecking Ball

Hold It Against Me is the only one that's completely forgotten in that bunch.

None of these are exactly forgettable, Part Of Me, Hold It Against Me, We Are Never Ever Getting Back Together, We R Who We R, Problem, seem the most forgotten of the ones listed though.

Hold It agaisnt Me
I Knew You Were Trouble
Blank Space
also Wrecking Ball and We R Who We Are

WANEGBT definitely

We R Who We R. I didn't even remember this was number one

We R Who We R. Don't even know how it goes.

In order from most forgotten to most remembered:

Forgotten:
Hold It Against Me
We R Who We R
Born This Way
Part Of Me
We Are Never Ever Getting Back Together
Just Dance
Tik Tok

Remembered:
I Knew You Were Trouble
Blank Space
Roar
Shake It Off
Problem
Hello
Firework
Wrecking Ball

For those voting Part of Me:

Youtube daily views

May 28: 419,137
May 29: 346,701
May 30: 374,072

Despite being 4 years old.
Could you explain me?

It's a tie between We R Who We R and Part of Me, with the latter having the slight edge.