We haven't seen the full extent of Ceph technology (in Crysis: Legion the Ceph in New York were described as gardeners rather than soldiers), so at this point it's hard to say. Judging by what we have seen, though, the Covenant would squash the Ceph easily. Covenant soldiers often perform EVA operations and they have dealt with infectious spores in the past (i.e. the Flood), so biological warfare that's non-contagious would quickly become useless against the Covenant, once they realize what's going on. The Covenant have highly damaging plasma-based weaponry and various others as well, while the Ceph are limited to ice-based weapons (which are no better than bullets against energy shields). Ceph heavy weapons don't fire rapidly, while the Covenant have heavy weapons that can fire as many as 5 shots per clip. Add all this to the fact that the Covenant can destroy the surface of any planet via orbital bombardment (aka glassing), and you have charred squid remains.

The Borg.
"You will be assimilated, resistance is futile!"

(Both Covenant and Ceph would be assimilated, used and abused!)

They are both equally lame, but the Ceph would win with their awesome biological weapons.

Umm i think covenant would win because we are forgetting hunters, jackals with a beam rifle, elites with energy swords, brutes with grav hams and the drop ship is way more powerful than the ceph airship and oh yh SCARAB. And banshees and wraiths and ghost and suicide grunts and revenants and oh the list goes on and on. And oh yh fuel rod.

Covenant are not spread across hundreds of Star Systems they have limited resources trying to fight humanity. Read the book, the latter part... and you get the sense that the covenant will pail in comparison to the Ceph.

The Covenant has plasma and shields. The ceph has exposed goo in their back.

Gumby and pokey would be the greatest threat of all.

The Covenant has plasma and shields. The ceph has exposed goo in their back.

Before making those weird claims understating the Ceph ideas, you might want to read the book. It iwll explain why.

The Covenant has plasma and shields. The ceph has exposed goo in their back.

Before making those weird claims understating the Ceph ideas, you might want to read the book. It iwll explain why.
It was never fully explained in the book, but explanations vary from psychological impact to deceiving predators. The bottom line is that exposing the tentacles isn't necessarily a stupid choice. That being said, the Covenant has plenty of weapons that can kill Ceph easily, exposed tentacles or not (the cloaked Ceph at the end have protected tentacles). The Ceph are at a major disadvantage when faced with Covenant energy shields.

Its explained at the latter part of the book. Not talking about their tactics but what they are and how big their civilization are.