Depends on what style you like really. The earliest albums were very west coast influenced earning them the tag of the English Jefferson Airplane.
Although they used English folk songs earlier, the first real attempt at an English folk-rock fusion was 'Unhalfbricking', but the album that is the definitive English folk-rock album would have to be 'Liege and Lief'. 'Full House' is also a near masterpiece, although missing Sandy Denny's vocals, but in my opinion they lost most of their creative edge when Richard Thompson left, and their subsequent albums, for the most part, lack the originality to really lift them. There's still a lot of good stuff on them though, but to me they are just not as 'special' as the earlier ones.
I'm pretty sure their will be some differing opinions here though!