When I started mixing and matching songs from different albums to create "mixtapes" (playlists really) for myself, I did a deep dive into ReplayGain (because songs from different albums won't have the same levels). While Winamp had decent ReplayGain functionality (adding RG tags as well as reading them), Foobar2000 was the best.

Managing levels is very important if you're listening on headphones. Otherwise some songs that are too loud will really hurt the ears.