Hmmmn.
Well firstly the title song of Megaman 4. It's my favourite Title song Capcom ever made.
Also from 4; Ring Man, Bright Man. and especially Dive Man.
I didn't really like 5 but I really liked Stone Man's theme. ALso Wave Man bcause it meninds me of Slash Man ( I played 7 before 5)
Plant Man from 6, The final battle theme and the Wily fortress song.
All of the Megaman 7 soundtrack epecifically Freeze Man, Bass' them, and the password tune. Don't ask. =P
Grenade Man from 8. Also Duo's theme.
Cold Man from Rockman and Forte.
All the character themes from Battle and Chase and Duel (wily's stage course.)
Wire Sponge (from Xtreme or Extreme 2, I forget)
The overseas promotional theme for Maverick Hunter X. (the instrumental song that we gotinstead of Don't wanna be by Spinwake)
The Answer (opening for X6)
Crecent Grizzly's stage song.
Megaman X 8's OST minus Gigabolt Man O War's song...
A lot of songs from X6.
Zero's intro level from X7 as well as Tornado Tonion's.
Most of the character themes from Command Mission.
Determined eyes from Zxa.
The main theme for Starforce 2.
The minor battle song from Starforce.
A few songs from the battle network games like 3' main theme as well as 3's minor battle theme.
I know it's a lot but hey. could be a lot more I guess.