Who(m)ever you choose is fine with me.

Which is grammatically correct?

Whoever you choose is fine with me.
Whomever you choose is fine with me.

I only ask because it took me half an hour to finally settle it by consulting a PhD via grammar logs to figure it out, an answer said PhD initially got wrong too. Still as I type images of adjective clauses and subject verb object order bounce in my head. Really, it seems exceedingly silly to spend half a sticking houmr to figurme out whether or nmot my sentemnce had an extra "m" in it.

"The only time I wouldn't want to die is after saying "God if you exist, strike me dead!" Which I have done from time to time in religious arguments. I can easily come to terms with death. But whoever I would be arguing with at the time would take that as proof of God. That I wouldn't like. And, being dead, I wouldn't be able to argue that my death was just very coincidental." - Tatarize

Though there's still some debate that this sentence has a complex subject whereas the example in grammar logs doesn't.

UPDATE: So hour four of whether or not some word that could easily be replaced needs to have an "m" in it. RH3 counters by showing that whoever and whomever used as such need to be a subjective clause and thus the adjective clause stance should be disregarded.

Try this, then.

"The man with the bike leered at the waitress."

We can remove the adjectival phrase "with the bike" to get

"The man leered at the waitress."

That's OK. We have the same declarative sentence, but with a little less detail.

Now do the same with

"Whoever I was arguing with would take that as evidence for God."

Removing " I was arguing with" gives us

"Whoever would take that as evidence for God."

And that is not an English declarative sentence. It has the form of a question.

"Whoever would take that as evidence for God.?"

This suggests that "I was arguing with" is not an adjectival phrase.

Also noting,

"George is wrong."

OK.

"Whoever is wrong."

Nope. "Whoever" has to be part of a subject clause. It is in all your examples.

UPDATE: Hour 8 of debate over the letter "m" in a sentence from several years ago.

RH3 made a pretty good argument and proper citations in favor of whomever and I'm now inclined to believe them. And I suppose since his university pays him to know such things, he's probably pretty well versed with such things.