Without a doubt ERA II is his best lib. If you can pick up a second one beyond that honestly it really depends on your tastes and needs. If you're looking for vocals, Vocal Codex is wonderful. The Celtia patch there IMO is one of the most beautiful solo vocal VSTs in existence. I get a lot of use out of the Heroica patch as well. Shevannai is a really nice lib and has a bit more functionality than Celtia ... though IMO the sound in Celtia is easily better. So it depends on what matters more to you. In case you would also consider a non-vocal lib and have an interest in more "eastern" sounding instruments, Ancient Era Persia is excellent.