Depends on mood. Some days I like Beethoven's Pastoral; some day I prefer Chopin's Prelude Number 4; some days Night on Bald Mountain; and some days only Bach's Tocca and Fugue will do.
Other days, it's Ravel's Piano Concerto in G. Then again, it could be Rachmaninov's 3rd Piano Concerto. Or Takemitsu's From Me Flows What You Call Time. Then again, how about Dutilleux's Metaboles? Or his Shadows of Time? Or Vaughan Williams' Lark Ascending?
Today, it was Ravel's La Valse. Tomorrow - who knows?
That's tough. Honestly I really can't pick just one thing. Just about all of the above could qualify, and many more. I'll pick a "favorite for today" - a little Smetana. I'll say, I am pretty partial to the Romantic Period.
It might be cliché, but the first movement of Piano Sonata No.14, Op.27 No.2 (Moonlight Sonata) by Beethoven. I would post a link to it, but I'm just not finding a recording of it on YouTube that is played in the manner that I prefer, with a tempo around 44 bpm.