Hollywood's scariest night was chock-full of costume and make-up-wearing celebs ... but it was Paris Hilton who really turned heads when she walked in as Britney Spears!!!

Paris hit L.A.'s hottest Halloween party -- Casamigos -- Friday night with a slew of A-listers, and she stood out from the pack in her sexy blue stewardess outfit just like the one Britney wore in her 2003 music video for her hit song, "Toxic."

Given all the Britney news -- and drama -- of late, Paris gets an A+ on her Halloween report card for choosing to appear as a super relevant pop culture figure.

The rest of the guest list was also frightening in its star power. Feast your eyes on these bold-faced names ... Justin Bieber, MGK, Megan Fox, Ed Norton, Bill Maher, David Spade, Cindy Crawford, Rande Gerber, Austin Butler, Kaia Gerber, Tobey Maguire, Len Wiseman, Brett and Jason Oppenheim, Rachel Lindsay, Brock O'Hurn, Tyga, Saweetie, Alessandra Ambrosio, Paris Jackson, Molly Sims & Scott Stuber, Lauren Conrad, Charlotte McKinney, Sara Sampaio, Rachel Zoe & Rodger Berman, Robin Thicke.

Some of their get-ups were pretty freaky, too ... MGK and Megan went all Quentin Tarantino, dressing up as two bloodthirsty murderers from his "Kill Bill" movies.

The party was killer as well. It was held in a private house in Bev Hills and was packed with costumed revelers.

Everyone was drinking booze from the open bar and munching on tater tots, pizza, grilled cheese and sliders. We're told the vibe was great all around and people had an awesome time.

But don’t forget kiddos … Halloween doesn’t officially kick off until Tuesday.