Favorite ever is tough because there are so many.
The P-51 certainly stands out.
Of those I’ve flown, I think it’s a toss-up between the Icon A5 and the T-6A.
Once you get used to it, the T-6 is simple to fly. It feels like it has unlimited power, it’s very agile, has great visibility and air conditioning, and plenty of gas. I’m a sucker for a big prop, and it requires a pilot to actually use the rudder.
The A5 is just a superb design…ergonomically, aerodynamically, and hydrodynamically. It’s mostly a toy, though it may be the best training aircraft I’ve ever flown.