This is really hard, because every K stage has around the same amount of songs I like...
K2:
- Kinjirareta Futari
- Blue Rose
- Virgin Love
- Korogaru Ishi ni Nare
K3:
- Ki ni Naru Tenkousei
- Maria
- Kimi wa Pegasus
K4:
- Saishuu bell ga naru
- Oshibe to Meshibe
- Melos no Michi
K5:
- Saka agari
- Hitei no REQUIEM
- Mushi no ballad
- Wagamama na nagareboshi
K6:
- Reset
- Seifuku no Resistance
- UHHOUHOHO
K6 is definitely the flavor of the month, but overall I think K3 takes it.