My Opinion 
Buy:
+ Master of the Rings
+ The Time of the Oath
+ Better Than Raw*
+ The Dark Ride*
+ Keeper of the Seven Keys - The Legacy*
# Keeper of the Seven Keys - Part 2
Consider:
# Chameleon
# Keeper of the Seven Keys - Part 1
+ Rabbit Don't Come Easy
+ Helloween - High Live
Avoid:
# Pink Bubbles Go Ape (it does have one or two ace tracks, but most of it is tripe)
~ Walls Of Jericho
*= Exceptional Album - A Must Have!
+ = Andi Deris on Vocals
#= Michael Kiske on Vocals
~= Kai Hansen on Vocals
Hope that's of some help
Andy