Some years back, there was an article in Kitplanes, I believe, asking three writers this same question. Two of the three said RV-4. That was my second choice behind the Thunder Mustang, chosen because it's ailerons didn't get heavy at speed. At least, not up to the mid-200s.
Of course, there's more than handling as airplane criteria -- performance, ride in turbulence, maintainability, interior comfort, visibility, etc. Not that the RVs don't do well, there, mostly.
RV-9A at KSAV (Savannah, GA; dual G3X Touch with autopilot, GTN650, GTX330ES, GDL52 ADSB-In)
Previously RV-4, RV-8, RV-8A, AirCam, Cessna 175
ATP CFII PhD, so I have no excuses when I screw up