I think this is based on a prejudicial, Victorian insistence that, "a sentence should never end with a preposition".
If you remove "the most" it makes it easier to see we would then be in grave danger of committing this 'crime'.
Saving the planet is the topic which I am interested in.
Churchill is supposed to have responded to this after it was insisted on by a speech writer (in fact this is apocryphal), but the lesson stands, nonetheless.
He is reported to have said
This is the sort of nonsense up with which I will not put.
So, people feel more comfortable if the preposition is moved away from that 'dangerous end of the sentence' and put somewhere 'safer' instead.
Saving the planet is the topic in which I am interested.
All Victorians are now much happier, but in speech people don't construct an entire sentence before they start saying it, so that's where prepositions often end up.
For those of us less affected by this Victorian insistence, really the part that makes your sentence the most awkward is actually where "the most" fits. I would be perfectly happy with either
Saving the planet is the topic in which I am the most interested.
Saving the planet is the topic which I am the most interested in.
Although even I, as a non-Victorian, prefer the one where the preposition is moved to the less 'dangerous' location.