It's difficult to give advice because we don't know enough about how you plan to use the recorder. For example, what microphones are you planning to use? Will they be placed a distance from the recorder (making a balanced mic circuit more important)? Is your subject generally loud (say, many primates, amphibians) or quiet (most birds). How important is portability and battery life? Is water resistance an issue? And so on.
Here are some general comments. According to tests on this web page (
http://www.avisoft.com/recordertests.htm ), the Fostex FR-2LE offers the quietest preamp performance (-129 dBu) of recorders with XLR inputs, and price below $600. The Marantz PMD-661, at -125 dBu, is good, but you'll be able to hear that extra preamp noise in quiet recordings made with good mics. The older Marantz PMD-660, at -120 dBu, is another step down in preamp quality.
I've used all three of these recorders, and would agree with this ranking of preamp quality. The FR-2LE has its quirks, but it's a proven performer in field settings.
These three recorders are relatively large. Today's smaller handheld recorders offer decent performance at cheaper prices. But you give up the balanced inputs and locking connectors provided by the machines with XLR style connectors. For example, the new Roland R-26 has relatively quiet preamp (-124 dBu), but the XLR inputs are unbalanced and lack the locking feature. The Roland D-40 has locking connectors, but lacks balanced inputs. And, worst of all for nature recording, its preamp is noisy (-107 dBu).
The built-in mics in any of these recorders leaves a lot to be desired. So a handheld recorder is often paired with a pair of external mics. The small recorders can't provide the same sort of battery power to the mics, so you often have to use a separate battery box to power the mics.
Tell us more about your needs, and we can make better suggestions.
Flintstone