I bought a g21 not to long ago cause i'd never had a glock and their a lot lighter to pack around than a 1911. Think i like my m&p and xdm a little better.
I have a Glock 19 and used to have a Springfield XD9,but I'm gonna put a vote in for my Walther P22. It's seen more rounds through it than any other gun I own. It's been reliable and it's a lot cheaper to shoot.
I shoot the Glock's, Sig's, and S&W M&P and S&W 99 to name a few. Amongst the guns designed for concealed carry i would opt for the S&W 99 followed closely by the S&W M&P. I prefer the unique ambidextrous mag release of the S&W99/Walther P99 and the ergonomics of either S&W over the Glock or Sig 226.
Of the firearms listed I have the most experience with the XD line so I would have voted there. The M&P isn't bad but just doesn't fit me as well as the XD.