Probably the B'Rel Bird of Prey. Even though the D-7 is presented as the good old Klingon ship, and it's the first Klingon ship and cruiser I have seen, the B'Rel is the ship of great Generals and has been in service for decades, definitely a Klingon ship if it survived glorious battles with the Federation, the Dominion, and all who dare oppose the Empire.
However, it's a matter of taste, and I'm just giving out my opinion but I'll definitely choose the B'Rel as the iconic Klingon ship.
Well, I'm an old fart and cut my teeth on TOS, so I've got to say the D7 (and K'Tinga).
As far as iconic goes, check out the shirt the singer for Sto-vo-kor is wearing here: 'Nuff said.
Hi! I used to be JBHemlock. There's already one of those in PWE. I play the Klingon Kerr, but there's already a Kerr in PWE, too. So I'm Gehmor, my Cardassian alt. Just picture me with a plate of gagh.
I would say the BOP because it was the most used in the movies and later series, especially TNG and DS9. I remember when the B'moth was on DS9, I was like wow, they still use those. Of course, VOY would late show one too in Prophecy.
The D-7 is the boxiest, and worse "design" I have ever seen. It honestly looked like a second grader drew a picture and gave it to his father who happened to be in charge of creating the art of the D-7. Even then it looks "unfinished", as if the artist just stopped and went home or got fired after making a pathetic design.
The BoP screams "KLINGON" to me. I don't care if they "stole" the design, or if the script changed and they ended up "getting" the BoP. I don't care what movie or series it appeared or .. didn't appear in. I don't care what ship blew up what ship in what series during what time. When I see a BoP I will find myself biting my lip thinking "ok how many of them are out there ..." and wondering just what amazing idea the captain of the "inferior" BoP will use to knock a hole in a Galaxy class cruiser.
D7 - ...an afterthought of a "what could have been" design
BoP - Everything a Klingon represents and I play STO just to fly one.