Kett vs. The Reapers

Normally I'd say Reapers. Even if considering the Kett may be wide across much of the galaxy.

The Kett are the 'existing empire'. That doesn't have to be too much, even if it can be involved in future stories in interesting ways.

The Kett adopt desirable traits from exalted species, but we don't have the confidence they even have the desire to adopt traits that would help them against potential dangers - they could even be engineering themselves in a hurtful manner without realizing it. You know, the whole eugenics trap?

I think the Kett, as I understand them so far (most story complete, reading everything I can, 50% content complete) could stand a chance if:
-The fight was in Andromeda (no Reaper ambush, mass relays)
-The Reapers had to at least somewhat spread out (they did in ME3 but I'm saying even more)
-The Kett had something of them that could take on synthetic technologies and spread it wide across their people in surprising ways (like a Super Collector People finding a way to stave off Reaper control by becoming Indoctrination-resistant through their biological side)
-


The thing about the Kett is that they're probably less of a current threat than MEA's plot presents (and its only the weakened state of Heleus/Initiative/Angarans that makes them so dangerous), but they're possibly more of a future threat depending on what they actually decide to do. Their lower rungs may have even less say than a regular grunt in more normal armies, but their upper rungs may have enough sense to see what their enemies are doing and adapt to them amazingly.
Andromedans don't seem to find AI unknown - just the Angarans have their mystery about it, and the Kett seem to think that they don't require it (yet). But now they might, and the combination of it and Kett physiology could produce a much better and even diverse and unpredictable (once you figure the Reapers out, their method, while maybe terrifying, is expected) danger.