In the sentence "You changed the name! Oh I'm so proud. Thanks to whomever was responsible for this!" Is whomever or whoever the correct choice?
It should be "whoever"
The case is determined by the subordinate clause, not the main clause.
Thanks to whoever was responsible for this!"
Site Hint: Check out our list of pronunciation videos.
To determine whether to use whoever or whomever, here is the rule:
him + he = whoever
him + him = whomever
Examples: Give it to whoever/whomever asks for it first.
Give it to him. He asks for it first.
Therefore, Give it to whoever asks for it first.
We will hire whoever/whomever you recommend.
We will hire him. You recommend him.
him + him = whomever
We will hire whoever/whomever is most qualified.
We will hire him. He is most qualified.
him + he = whoever

Hope the above helps.
Thank you!