Best: Sisko in In The Pale Moonlight. (But also the worst
That wasn't diplomacy, that was dirty tricks.
But it was made necessary by one of the most catastrophic failures of Fed diplomacy: bungling the wormhole situation with the Dominion.
Dominion: don't use the wormhole, the GQ (or at least the part of it the wormhole leads into) is our territory, which we plan to defend with maximum violence.
Feds: STFU, we do as we please. Bring it on, gumbies.
Romulans: WTF, what about us? if the Doms get mad at you, they won't be too selective who they hit. We're blowing up that wormhole before anyone, namely us, gets hurt.
Feds: Those sneaky Rommies! We sure put a stop to their nefarious scheme to protect the Alpha Quadrant.
Romulans, to Cardassians: The Feds are being their usual idiotic selves. This leaves no choice but to handle the situation on our own by a first strike, thus proving that they're not the only idiots around.
...and we all know what happened as a result. The gumbies must have wondered what they did to deserve a quadrant full of Keystone Kops as neighbors.