Heheh, what really got me into OSTs was the opening song from DBZ, you know the one with teh catchy 'duh duh deh duh duuuh duh, duh duh duuh duh.'.., (well the guitar riff).
Yuko Kano's OS' from GITS were really what got me into them. like "Inner Universe' and 'rise. Here singing on this series was superb. loveed the opera accents in her voice.
A classic one would have to be more like the imfamous 'Shell' from Witch Hunter Robin. simply good musically wise, with nice vocal rythms :P
And finally 'Aozora no namida' from Blood+
All these ost's now provide me with nostalgic feelings, which always make me remind myself of the good ol' days just years back. XD