"Like a Virgin" and "Oops!... I Did It Again".

impactful? none...
some of them are just iconic and some of them are not rememberred

Mess. Let your stan cards OUT for a moment.

Like A Virgin trumps all. But Since U Been Gone is one of the biggest songs of the last decade. Its impact can't be denied.

Like A Virgin > Oops > Since U Been Gone > California Gurls > rest

Technically, Breakaway is the lead single tho.

Most of these aren't impactful at all. I'll go with...Like a Virgin.

#1 Billboard summer song California Girls

Like A Virgin and Oops!...I Did It Again

I voted for Oops! because it was really impactful and it's still remembered, also voted Like A Virgin for obvious reasons and Born This Way because it's a gay anthem now and the phrase will be used everywhere in gay parades/events for decades to come.

Some saying none
Esp denying oops

Hypocritical considering that without that song, some of your faves wouldn't be here today cos that song cemented Max Martin as the to go to hit maker he is today.

Britney Spears - Oops!...I Did It Again (2000)

Like a Virgin.

California Gurls is a close second tho.

BTW ''Since U Been Gone'' is NOT the lead single of her second studio album.

LAV followed by Oops

Oops, Rehab, Since U Been Gone

SUBG

Hand On Your Heart of course

Seriously though, it's probably Oops! or Like A Virgin.

OIDIA
Like a Virgin
BTW/ Bad Romance

It is considered as the lead single from Breakaway tho.

"Breakaway" (the song) was released as a soundtrack for the film The Princess Diaries 2: Royal Engagement.