As much as I also like the heavies, I'll be the unique snowflake by voting for the Bf 110. I love the heavy fighter concept and along with the American P-38 Lightning, the Zerstörer is among my favourite WW2 airplanes.
You can also use the Bf 110 as a bomber (both dive and level), so it would be a versatile addition to the WOTR planeset.
Besides, it was also important as a night fighter. And that is something never properly implemented in a WW2 flight sim!