I've got three that come to mind, and I'm curious about what experiences others have had! I suspect this is going to be heavily dependent on flying/piloting style, of course.
In no particular order...
#1 - Pfalz D.IIIa - Always overshadowed by its Albatros and Fokker contemporaries, and unjustly so in my opinion. I've learned to treat the Pfalz as an underpowered German take on the SE5: a rugged, stable gun platform that excels in boom-and-zoom dive attacks and high G maneuvers. I like to drive my aircraft hard, and this is the plane to do that; this thing eats Split S's for breakfast, and doesn't break a sweat doing dives that reduce other aircraft to sawdust. I only wish it were a bit faster in level flight.
I've heard about the Pfalz being stall-prone, but to me the handling feels pretty smooth in the most realistic flight model setting. And even when it does stall, it's tended to stall straight forward for me.
Honestly, I think had Pfalz stuck with this design, and maybe thickened the airfoil a bit (like Fokker did with its later designs) while using the 200-ish hp Mercedes and/or Benz engines in use in 1918...that would have given the Fokker D.VII a run for its money. That kind of "Super Pfalz" would have come very, very close to realizing the potential of "Germany's take on the SE5a" while serving as the best dive attacker of the war, even more so than the actual D.XII that Pfalz put out in 1918.
#2 - Roland C.II - Not even a fighter, but I've had campaign pilots become aces in this thing because it's simply faster, more rugged, and so much more
smooth in its handling than anything else on the German side for most of 1916 - and even going into 1917, the Roland is still more than a match for Allied two-seaters. (And, like the Pfalz, it's a pretty solid gun platform). I'd certainly take this over any Eindecker, and while the Albatros D.I/D.II surpass the Roland in some respects, the Roland still has *much* better visibility.
I really don't get why the C.II didn't catch on, when IMO it performs so much better than the DFW C.V that would go on to become the standard German reconnaissance two-seater.
#3 - Nieuport 28 - IMO France chose the wrong plane when they rejected the N 28 because "we already have the SPAD 13." Unlike earlier Nieuport designs, which I often find to be somewhat "twitchy," this is yet another smooth flying experience. It's not as rock solid stable as the Pfalz or Roland, but it's more than stable enough, and it's so much more agile than anything the French are flying in 1918 - all while still moving at 128 miles per hour!
While I wish it could dive a bit harder, this is one of those aircraft that does a little bit of everything: this design maintains ~95% of the positives that I care about in the earlier Nieuport line, eliminates ~95% of the annoyances from those designs, and packs ~95% of the speed of the SPAD 13 and SE5a. I can certainly understand why some real-life pilots in the American squadrons were upset at having to switch from the N28 to the SPAD 13.
(Plus, I have to admit that Nieuport found a very elegant solution for mounting two guns in rather tight quarters).
I think this is one of those aircraft that just wasn't given a chance to move past the initial growing pains that many classic designs went through. Had there been more opportunity to tweak some of the details a bit, it wasn't going to be that hard to address the N28's diving issues. I've even read about frontline mechanics for the American squadrons who were able to make the necessary changes on their own - just imagine what an actual factory could have achieved.
Any others?