Best/greatest isn't the same as favorite imo. Personally, I love classical but I'll listen to metal most of the time because I can't enjoy classical if I am not fully invested into it (I don't want it to be in the background). Classical is the most challenging and rewarding music when you pay your full attention to it and listen to it many times but that's hard to do most of the time!
Sorry about the insane rant, but whenever this subject comes up, I can't help myself.
This is probably an unpopular opinion, but personally, I've always rejected the notion that your favorite thing is necessarily the thing that you can pick up and enjoy at any point/most frequently/most easily (guilty pleasures that have an unwanted hold on you would qualify, if that were the case).
If you appreciate it more and have come to the conclusion that classical music is capable of giving you the most rich/satisfying/rewarding experience when you give it your full attention, I don't really see why it should be determined less of a favorite just because it's difficult to find yourself in the necessary head-space for it. If I were you and I felt the same way about the two genres, I would still consider Classical more of a favorite than Metal.
Even taken to the extreme of something that you appreciate more than anything else based on a first experience, but find impossible to revisit, I think that would still qualify as "favorite", personally.
Nobody seems to agree with me on this, but in my opinion, while best and favorite isn't TECHNICALLY the same thing, a person's opinion of it should be more or less interchangeable in practice because "favorite" is the thing you CARE most about, not the thing that you can go back to at any time/all the time, and "best" is the thing you APPRECIATE most (which usually stands to reason to also be the thing you care most about), not the thing that you can make the best dispassionate objective argument for about it's technical merits, IMO.
More accurately, "best" is probably the thing that objectively/hypothetically CAN be/deserves to be appreciated most, but I don't think that a person can determine that with certainty beyond the degree that they themselves have learned to appreciate it, IMO. Reaching beyond that has always felt like cheating/guesswork/being presumptuous, to me.
I suppose it's hypothetically possible to care about something else more than the thing that you actually/genuinely appreciate most, but my mind sure as hell doesn't work that way, nor can I really wrap my head around why it does for others.
For these reasons, I've never ever bought into the idea that "Oh, I totally agree that this is the best thing, but it's not my favorite" or "Oh, this is my personal favorite, but not my opinion of the best", personally. For me at least, if I found myself saying something like that, it could only be an insecure cop-out/self-deception of what I truly think, on my part. I can't separate the two.
I mean, the way I see it, the only reason I have subjective differences in preference from anyone else in the first place is because I disagree with them about what's true and have found certain elements that I do appreciate to be better or more important while finding other elements that I don't appreciate to be worse or less valuable. Doesn't mean my experience is necessarily right of course, but I'm as right as I can possibly know, IMO.