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I gotta go with Catachan. I don't think I've ever seen Fenris listed in any official literature as a death world at all, only as a feral world unless this happened with one of the last re-writes.
One on one both Fenrisians and Catachans are gonna be tough as nails, pushing human limits.
In a cage match with primitive weapons a Fenrisian might have the edge of experience with the fighting style (although I'd never underestimate a Catachan with a knife) Put them in the jungle though and a Catchan will F him up with boobytraps and such before he knows what hit him

your avatar is a space wolf emblem Purplefood. No one is going to convince you of this but Fenris is all bad to a Space Wolf player. If you don't have a Space Wolves codex it's only a foot note. EVERY ONE in 40k knows Catachan is the original definition of a Deathworld!
-full disclosure, I don't play Space Wolves or Catachans although a few Catachan models slip into my Tallarn regiment once in awhile

I'm not sure of Baal, could be a death world. The natives live on the moons orbiting it (it's radiated but not I think immediately deadly). You say the Fenris deathworld information is in the latest Wolfie codex, so that's new retconning to me. Understand my game knowledge is rooted in 2nd ed and it was never implied then