Misternice,
English Rose is a single disc compilation and highly recommended IMO. You'll get a good taste of Green's style and a couple of highlights from Jeremy Spencer (especially Evenin' Boogie, a fantastic slide guitar workout) and Danny Kirwan (especially Something Inside of Me, a fierce blues rocker). The handful of Green tracks include the radio hits Black Magic Woman and Albatross.
If you want a broader sampling, I'd recommend Men of The World, a three disc compilation of Mac's early years. Lots of Green and a better sampling of Kirwan, too.
The first, self-titled FM album is a great set of blues rockers and Then Play On is a fantastic experimental blues rock album. Both of these show off Green at his best.
As to Green's solo material, it's pretty spotty. The songwriting just isn't as consistent and Skies (the one you already own) is probably the best of the lot IMO. In many cases, the songs aren't written by Peter Green and they just don't do the trick for me. The guitar playing is erratic, too, unfocused stretches followed by terrific moments.
BTW, Green's first solo record "The End of The Game" is really polarizing. One long jam that some find mesmerizing and others find infuriating. I love Peter, but I'm usually in the latter camp.
Good Luck.
