for most popular, brahms 1 is also fairly frequently played. so is beethoven 3. of course, there is prokofiev 2 and 3, bach d minor, ravel, liszt 1 and 2, schumann, saint-saens (no. 2 is especially often played, but I think no. 5 is the best. No. 4 is also a cracker). Gosh, there's so many more. For Mozart, I've only really listened to No. 9, No. 17 and No. 27 although most of Mozart always seems a bit predictable and jolly for me...
If I were you, I'd give the Liszt ones a listen. I think that's what I moved on to after Rachmaninov... then I went to Mendelssohn's and then Grieg (if I remember correctly!). then I started to listen to all the unplayed stuff... and only recently have I "retraced" my steps and started listening to Beethoven and Brahms.
The Hyperion Romantic Piano Concerto series is fantastic as far as unknown piano concertos go:
https://www.hyperion-records.co.uk/s.asp?s=S_1But I have to say, Medtner No. 2 and the Busoni are probably top of my list now. Along with brahms 1, scharwenka 1 and the henselt concerto (Raymond Lewenthal's recording of the Henselt is a belter!).