Actually, ALL of the MDB albums are genius, and ALL are worth buying.
But, for starters, you should buy the one that suits your particular tastes the best - and each MDB is pretty different than it's predecessor (except the last two).
"As the Flower Withers" (1992) - this is pure doom/death. Seven songs, ALL death vox; it's good, but is easily my least fav.
"Turn Loose the Swans" (1993) - simply put, it's the BEST album ever recorded by anyone in any genre of music. It's dark, it's progressive, it's angry, it's melancholic, it's beautiful.
The songwriting is, I feel, the BEST the band's ever achieved.
The lyrics are even perfect!! The violin & atmospherics are plentiful. The vocals are about 60% clean, 40% death.
This record is the one MDB album that should most appeal to OPETH fans!
"The Angel & the Dark River" (1995) - A very 'clean', yet sad, My Dying Bride. On this great album, the violin really takes center stage. Only 6 songs, but ALL six are among the bands best songs. The vocals are ALL clean. This is, perhaps, their most 'avant garde' release.
"Like Gods of the Sun" (1996) - Their most "mainstream" record. It's not mainstream in the "pop" sense, however. It's mainstream in that the songs are much shorter in length (averaging 5-6 minutes). The songs are very fluid & easy to get into. Again, the violin is beautiful, but this time, the guitars are much louder in the mix - it's very heavy! again, 100% clean vocals.
"34.788% Complete" (1998) - This is a very misunderstood record. Many hated it when it was first released, but now, are beginning to accept it's greatness. The vocals are all 'clean' again. Sadly, gone is Martin, the violinist. There are only 7 songs this time. This album has a very 'urban', futuristic sound to it. It sounds like Tool mixed with earlier MDB. A great record, but not their best. The lyrics are also very 'plain' - and not poetic/beautiful as the rest of Aaron's lyrics are. But that's not a huge big deal.
"Light at the End of the World" (1999) - After being lambasted in the metal press with 34.788%, the band reverted back to their older style - their very older style. This album is a mix of their classic era songwriting (Angel) with elements of the doom/death of their early says. These songs are much stronger than the 1st album, I feel. It's also long, doomy & depressive. And it's much angrier than their past 2-3 records. This record also marked the departure of long-time guitarist & songwriter Calvin. He is missed, but this material is still quite strong.
"The Dreadful Hours" (2001) - This album is basically a continuation of the style of "Light", only better. The songs are simply great, for the most part. The vocals are both clean & death - with each style being the best of singer Aaron's career. The title song is simply frightening as hell!!!
Anyway, I always recommend "Angel & the Dark River" to people interested in MDB, but since this is OPETH's board, I'd actually recommend "Turn Loose the Swans" as it's easily the record that OPETH fans would probably love the most!