Khloe and Kim Kardashian -- My Ass, Her Boob ... LET'S PARTY!

Khloe and Kim Kardashian were in a horse race Friday night ... each clearly trying to take the evening with assets that are undeniably spectacular.

Khloe and French Montana got nice and cuddly sailing on the Utopia III in NYC on the Hudson River ... to celebrate K.K.'s 30th birthday ... and her incredible ass clearly made some partiers go full mast.

And then there's Kim ... whose boobs could save perhaps a dozen lives in a nautical emergency.

Oh yeah ...Kris, Kylie and Kendall Jenner were there too.

Now that's a party we could get behind.