I think a lot of her Duty and I am... eras were the best. For example, I really love Endless Sorrow, M, and NEVER EVER.
SURREAL is a good example as well (even though I just had to look at the lyrics again). When considering it, I really do feel that is a good piece.
Eh, and maybe her RAINBOW era too, especially everywhere nowhere and Heartplace.
The song RAINBOW is good too. Of course, when considering these songs, I look at the PVs (if any), music, and lyrics.
Of course, these are what I feel right now. Perhaps there are masterpieces of the later eras that I just haven't heard of in a long time (of course, I haven't listened to all of NEXT LEVEL and RnRC songs...I know, what kind of fan am I? But they're never on youtube, and I have yet to buy the CDs...well, it is Christmas season.
).