This thread is a haven where you can go against the grain without fear of persecution. Let it all out.
The Classical Era is my least favorite era. I'm generalizing, but the diatonicism is banal to me. Generally if I can hum along to a melody the first time I hear it, it's not very interesting. I like chromaticism more as part of the harmony, as opposed to simple ornamentation. I think this era may have the biggest lack of depth as far as relevant composers go, i.e., after the greats the drop-off is pretty big.
Domenico Scarlatti is a giant of music who was ahead of his time. This probably wouldn't qualify as unpopular if his works received more attention. I've heard people say that Bach is the only Baroque composer worth noting.
Rachmaninoff's orchestral music far surpasses his piano work, including the concertos (though obviously there's overlap there). After Alkan and Liszt I don't have much use for that style of virtuoso piano music. Those symphonies, though, are exquisite.
Beethoven is long-winded. Still one of my favorites, but it's pretty tough to slog through a lot of those slow movements. I mean, we get it already. Move on. And then the repeats!