they are all great- i have to say it.
but, like most others id reccomened one of these 3:
Nightfall In Middle Earth- very very melodic/chorusy.based on the story of the silmarillion i believe its spelled. (lotr prequal). has "mirror mirror" and "into the storm."
Somewhere Far Beyond- older, a little more "thrashy" so to speak, thats the best word i can think of. a little more riffy i guess you could say. has "bards song" and "journey through the dark".
Imaginations From The Other Side- in-between the two. its got the melodic and riffy essence. i find that this seems to be the most favored album, mainly because not only is it used mentioned in top 3 reccomendations- but even those that arent BG fans seem to enjoy this album to say the least.
personally if you can only buy 2 cds, (assuming you only want 1 of those 2 to be a guardian disc), i'd say get Nightfall. its great, long, and really is a great introduction i feel. it was my first album as well.