presuming you aren't referring to jason(!) i know this sounds like a copout answer, but go for his greatest hits. I would advise his early stuff in sixties/early seventies before he went a bit mad.
IMO this is his best stuff (Jennifer Juniper, Colours, Hurdy Gurdy Man, Mellow Yello et)
I've got Golden Hour of Donovan on vinyl. It has all my favourite songs on it. Catch the wind,Universal Soldier,The Ballad of Geraldine,Catch the Wind etc. It's very good.