Its hard to say. Spits are "easy mode" anyone can get kills in them. P51s and P47s if flown in Boom and Zoom mode, again easy mode.
To push a plane outside its normal mode is the fun for me. Over the years I have changed from plane to plane.
I started in the F6F and have gotten close to 500 kills in it.
Then I decided I needed a bit more speed and jumped into the P38. I have close to 5000 kills in the 3, J,L,and G ("J" is my favorite of the bunch), but it got a bit easy so I changed again looking for a challenge.
First it was the Hog, just shy of 2500 kills in the main 3 1D, 1A, C (1D is my favorite).
Then it was the pony and the Jug, but I didnt do the BnZ much from alt, I liked to turn and burn with them. I learned from Yucca, Frenchy, and LilMak in the 47, and Fess, BigR and a number of guys that haven't played in a long time in the Pony. 2000 kills in each model with the P51d and the P47-D11 as my favorites in those.
Now a days Im in the 109. Mostly the G2 with a couple of jumps in the K4 now and then. Only 1200 kills in them, but I'm starting to figure out the aim and how to setup better shots.... as you might have noticed yesterday
Basically I think everyone's "favorite" will change over time if they stick around long enough. Everyone starts with the spits because its easy to be successful in and then interests will dictate what plane you jump into next. There are exceptions. SOme players like Fess and a couple of the "Chickentails love the pony and stick with it always. AKAK and the "Luckystrikes guys stick with the P38. the 56th guys stuck with the P47. The BT guys in the yak3. But I think the better players in the game are good in any plane they jump into and for me the variety is one of the things that make it fun for me. Kills counts are cool, but give me a good fight and win or lose I'll take that any day over score/kill counts.