I'd say to try every song on their "best collections".
Every Best Single+3 and Every Best Single 2. All VERY good songs.
http://downloads.khinsider.com/game-...-best-single-1
http://downloads.khinsider.com/game-...-best-single-2
But as for specific songs..
I recommend:
Feel my heart
Dear My Friend
Deatta koro no you ni
Shapes of Love
Time goes by
jump
fragile
KIWOKU
Sasayaka na inori
Grip!
Free Walkin'
FUNDAMENTAL LOVE
mata ashita
Samidare
azure moon
koibumi
good night
HI-FI MESSAGE
Koi wo shiteiru
As for albums, I'd say Many Pieces is a fan-favorite. Considered one of their best albums.
If you need anything uploaded just ask.