I have just started out on the A-10 last night, and found it to be really nice, and as stable as the F/A-18. It's a couple of days where I have been working on landing with the AV-8B Harrier, and so far, I have been really pleased with my progress.
So, if you really want to master an aircraft, I'd do the Harrier. The F/A-18 is pretty easy really.
Bro, seriously? I didn't know that. Why is it called Pegasus?