Jeng-
Fair enough question!
I fly all the a/c, though I tend to try to fly the plane that best suits a mission profile.
I seem to wind up defending bases against land-grabbers (hehe) so I do many anti-buff sorties- F4U-1C.
Of no base defense would be without its share of Nikki's, C205's, and Spits.
If I go for a lone-wolf patrol, I might grab a 109G2 or an La5.
Long range escort demands a P51.
Furball busting is a blast in the 190.
If you want to strike a base hard and quick, the B26 is the bird. If you have the time, a B17 can be unstoppable (given the time to climb).
NOE base captures can only be done in C47's.

Area of expertise? That's a hard one... I don't think I rise to the level of wizard in any of them; there are any number of people that are far better in any particular craft. My niche would probably be in the ability to fly any and be successful, and to be able to relate that experience to others.
When people ask me for help, it usually runs along the lines of "can you show me any tricks in the xxxxx?" The followup remark ususally goes something like "I can't seem to keep people off my six" or "How do I defeat someone diving on me from above?". My approach is usually to begin with some merges to see what that person might be doing. From there we can discuss merge tactics and such. We can progress to more advanced strategies after this point. The goal is to assess and develop (if necessary) the basic skills required for combat.
Now there certainly will be accomplished pilots that will come and ask for particular hints in specific a/c. I believe the best approach would be to discuss our experiences (what we've done and seen done) and experiment. I'm more than happy to be a target drone for practice!
