and are the duo we didn't know we needed ... until now, that is -- 'cause they came together in the most epic of ways, with mutual adoration for the other.

Check out this clip that's making the rounds ... you see RiRI fangirling hard after running into Natalie in Paris -- where she tells her she's the "hottest bitch in Hollywood." NP ain't shy about dishing out the compliments either ... 'cause she's clearly a fan of Rihanna as well.

Rihanna shamelessly makes it crystal clear she's well acquainted with ALL of Natalie's acting resume ... in particular, commending her abilities for pulling off the perfect innocent impression. Yeah, she's not kidding about being a fan -- you know she's seen her work.

Natalie, for her part, is pretty taken aback that RiRi is star-struck ... saying she may faint by all the love being poured on her. Like we said, she's certainly familiar with Rihanna too.

It's pretty funny to watch -- usually, Rihanna has a cool, calm, collected demeanor about herself in just about everything she does .

.. but here, she's like a kid in a candy store!

The exchange doesn't end there ... Rihanna asks someone to take a snap of them, to which they happily obliged. The two mega stars posed for a pic ... flashing big smiles.

Yeah, she's probably gonna frame that one ... maybe ditto for Natalie.