Brahms, then Chopin, then Beethoven.
I'm just very curious how Brahms played Brahms. I have a recording of him playing one of the waltzes (don't remember whether it was piano roll or early victrola), and it's very intriguing. Just so curious about a lot...like what sf meant to him. I think for Brahms, and maybe for all composers, sf means "make this stand out" not "suddenly loud". Lots of ways to make something stand out from what's around it. I'm curious about his tempos and a lot of his markings, like the beginning of Op. 119, No. 2: Andantino un poco agitato & then at the beginning "sotto voce" and "dolce". He does this a lot -- very specific directions that are somehow at the same time very vague. Like words just can't describe what he wants. I'd just like to talk to him about it.
Then Chopin, like someone mentioned above, to hear the rubato, and to see what the dynamics sounded like when he played (I'm guessing notched down from what we're used to).
Then Beethoven. No reason, other than he's the master composer, IMO.