I was a big Gandalf fan up until around the
STANDING STONES release as my taste must have changed (couldn't have been his music, oh no...

) and it went more in a prog-rock direction, whereas his music went more newage-ish than it already was (the electronics were getting down-played, which didn't help). But up until that, I loved most everything he did, as there were some releases that were kinda "contractual obligational" in their sound or just too 'precious' (Gandalf & Galadriel). But you just can't go wrong with the epic
To Another Horizon (got it playing right now and still love the doom 'n gloomy 'Tron & synths in '
Natural Forces Getting Out Of Control'),
Visions, Magic Theatre and
Journey To A Long Forgotten Kingdom. If you are looking for his most Tangerine Dream-ish one,
The Universal Play is the one that you want to check out.