At first glance, you'd say it's a close race, but for me, the HR Giger-inspired xenomorph comes out ahead, if only because it's less human than the Predator and as such, features many more entirely foreign giblets. They're still both pretty gross (so sorry if you were eating a late lunch).
These illustrations, the work of artist Brad McGinty, are a "homage to a crazy book called 'An Anatomical Guide To Monsters'", which feature "cross-sectioned views ... of the most famous Kaiju monters of 1967". McGintly goes into more detail on how he came up with the Alien and Predator pieces on his website.
Prints and T-shirts featuring the sliced-up creatures can be purchased from McGinty's site, but if you just want to view them in expanded form, both works can be found in full below.
Images: Brad McGinty