This episode belongs in the top ranks because - like other DS9 episodes in its class - it asks you to think instead of doing the thinking for you.
Memory Alpha has a full summary here.
The ends do not justify the means. No matter what Sloan says, a Roman maxim shouldn't be the final word when it comes to how we conduct ourselves when facing an existential threat. The debate over what is and isn't permissible in wartime should always continue because it is that debate which keeps us from losing sight of our humanity. At the same time, however, this is a fallen world. It is unrealistic to assume that your allies will never betray you (see also: the Soviet Union after WWII) -- or that everyone will play by your rules (see also: Islamist terrorism, which deliberately targets innocent civilians). If we could see the blood-curdling intelligence reports that cross President Obama's desk on a daily basis, I think we would quickly understand why he hasn't closed Guantanamo Bay (among his other broken campaign promises in re: the War on Terror). It's not quite so easy to make glib pronouncements on Bush's approach to foreign affairs now that you've seen what's really going on, is it, Mr. President?
Bashir puts up a fight for peace, cooperation and fair play in this episode, and for that, he should be commended. We like those things here at Right Fans and believe they should be our defaults. But - and this is why Ron Moore's script merits praise - Bashir is not the only character whose position is portrayed in a sympathetic light. Via Sloan and Admiral Ross, the audience is also made aware of the dangerous political realities that render Bashir's brand of starry-eyed idealism impractical. And it's deliciously subversive, we think, that Ross in particular is not cast as a villain at the end of this episode for helping Sloan -- that, instead, he maintains his position as a respected member of the brass. This indicates a recognition on the writers' part that a hero can be compromised and yet remain an admirable person. Nuance, my friends! It's what's for dinner!
Moore avoids comfortable prejudices and focuses on writing a damned good story in which every character's motive is immediately comprehensible. Bravo!
None of the performances are "holy cow!" amazing, but they are quite strong nonetheless.
In the Pale Moonlight, in my opinion, was a much more powerful presentation of the same general idea (i.e., that war is complex and messy and even the heroes don't always come out smelling like roses). But still -- I appreciate the reiteration.