I like the Manfred Weiss aircraft, and I tyhink you are spot on when you say they often get overlooked. Perhaps a thread on the smaller nations aircraft of WWII is in order? Maybe just answer in here?
Lithuania had the ANBO VIII attack plane designed by A. Gustaitis. Two-seater low wing monoplane, designed as an attack plane or a light bomber. The prototype was tested on September 5th, 1939. The engine – 930 HP Bristol "Pegasus" XVIII with two-stage air compressor. The occupation of Lithuania prevented the serial production of this last type of ANBO airplanes.
The Czech N.35 and B.135 look pretty modern for 1935 and 1938, and the B.135 had a top speed of 342 mph … pretty good! They only made 2 B.35, but made 47 B.135s. The Bulgarians only managed to get 12 and scored a victory against a B-24 with one. The Letov S.50 was decent for its time.
Estonia flew the Aviotehase PN-3 in 1939, but they only built 2 of which only 1 flew. Pretty good performance for a light fighter-trainer.
France isn't eastern European but is often overlooked when it comes to WWII aircraft. Breguet built the Br.46, .482, .690 - .695, .730, .731 (post-war). CAO (later SNCAO) built the model 200 in 1939. Caudron built the C-714, 720, 760, and 770. Delanne built the 20 T in 1938. Dewoitine built the D.342, D.500/510 series in the early 1930s, the D.513 in 1935, and the D.520 in 1939. It showed great promise, but was a non-player after the Germans took France. They continued with the D.500, D.720,.730, .731, .750, and .770. Farman fielded the NC.223. Hanriot built the H.220 / 220-2 and .232 that had decent performance. They continued with the NC-510 and 530. Latecoere had many decent aircraft. Loire built several decent seaplanes. The Loire et Olivier LeO 45 was a decent bomber until France fell and they also had several pretty good flying boats as well as the LeO-451. Loire-Nieuport built 3 very interesting planes. I particularly like the LN.161 with the Hispano-Suiza of 1935 that was absorbed by SNCAO, but they only made 3. Morane Saulnier built a series of aircraft, but the MS.46 was pretty much sort like a Hurricane. The Payens were nothing if not plain old weird, but seem to have had decent performance for the power. I have not seen flight reports. Potez and Potez-CAMS made some interesting aircraft. The Potez 230 looked interesting and was fast for the power, but they only made one. The Romanos and Roussel's llok good but are obscure. Likewise the SNCAC WWII planes. Some of the SNCAO planes look decent. The model 700 looks decent for the time. SNCASE only has one offering and it didn't make it. The SUD (later LeO, later SNCASE) SE.100 of 1939 looks plain weird, but the performance wasn't bad. Have never seen landing gear like that before or since. The SUD Ouest (Bloch) 175 wasn't bad, just didn't have a chance.
Bulgaria built the DAR 10A and 10F, both not bad for less than 1,000 HP. The KB-6 and KB-11 of 1940 were decent but they only made 24 KB-6s and one KB-11. I don't mention the Lazarovs only because they were slightly post-war.
The Swiss Doflug D-3802and 36803 look very intriguing for just-post-war. The Pilatus P-2 was a decent trainer and just made WWII, flying on 2 Apr 1945. Our museum has one in flyable condition.
The Swedish FFVS J22 looks like a dandy little fighter, and there is one in restoration right now that is supposed to go back into flying condition. The SAAB's look weird, but performed well. They followed this trend right into the Jet age with the Tunan that looked fat but out-performed the F-86.
The Dutch Fokker D.XXI and XXIII are wildly differing aircraft, but seemed to have performed well for their designs along with the G.I. The Koolhoven FK-52, 55, and 58 looked pretty good but they only managed to build 13 FK-58s before France fell.
In Poland, I'll forget the L.W.S. Zubr since it was a complete and utter failure. Some of the PAL's showed promise, notably the P.37 and P.38, but were never a factor. Likewise the P.50 showed promise.
Romania flew the IAR-39 biplane in 1939 but flew the IAR-80 monoplane in 1938! Go figure! The IAR-80A, B, and C were formidable adversaries. Ask the B-24 pilots hitting Ploesti. The IAR-81, 81A, 81C and 81M were even better.
Yugoslavia flew the IK-2in 1935 and the IK-3 in 1939 but only made 12 and 13 of each. The Ikarus Orkan bomber looked pretty good for 12935, but they only built 2. The Rogozaraki's look good but were not a factor. The UTVA Aero 2 was a decent trainer, later was Ikarus.
Hungary built the Mavag Heja II of 1940 and it looks like a Seversky. It was decent for 1,000 HP or so, if outperformed by contemporaries. Manfred Weiss built the WM-23 that looked decent for a 1,000 HP unit.
From Belgium, I like all the Renards, but have little in the way of flight data on them. The R.37, .38, and .40 look very good for the time. The SABCA S.47 looks dated even for the time.
The Austrian Skoda-Kauba SK 257 looks pretty darned good for a low-powered fighter-trainer but never went anywhere.
In Latvia, VEF or Valsts Elektrotechniska Fabrika (State Electro-Technical Factory) designed by Irbitis, the I-16 looks dated but the I-17 trainer looks OK.
Finland had some interesting planes, probably the most interesting of which was the VL (Valtion Lentokonetehdas) Pyorremyrsky that was supposed to be as good as Bf 109. They made ONE!
I can't verify the Marton as real yet.