That's like asking which of your two children is dearest to you, honestly. If we were talking about a band that's inconsistent then it's very clear cut which album between a couple is the best but in this case I can't decide for you because it's quality all around. Fragmentary Evidence has quite a bit of polish and intrigue, but then so does Concealed. It's sort of like any good progressive band like Opeth or Cynic: it's worth hearing every album because the focus is so different on each album and which is better is all a matter of preference. Most people are willing to admit that every song they produce is still a quality piece of work regardless of whether your favourite is Morningrise or Still Life and it's hard to deny that otherwise.
That said, I prefer Fragmentary Evidence but the same may not apply to you. It focuses less on the operatic vocals and more on polish fleshing out their different styles (jazz fusion/folk/brutal technical death). Also, one of the songs near the end of the album has one of the coolest segments in a death metal song I've ever heard that really catches you by surprise, but I won't ruin it by telling you what it is. You'll know when you hear it!
Hope that in some way helps you decide, haha, but it really is a tough decision for me to try and make for you.
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