Asked and answered two weeks ago:

http://line6.com/support/message/342958#342958

As of the most recent firmware update the HD Pro allows CC control via MIDI, opening up the door to many possibilities.  Otherwise the FBV Shortboard MkII is probably the best candidate.  Some people might recommend the original FBV Shortboard for the larger display (though it cannot toggle in and out of set list mode) or the FBV for its two expression pedals (though you have a lot of extra buttons that won't do anything).

From Line6Hugo in the same discussion:

The FBV Shortboard Mk II is the best controller to use for the POD HD Pro.

...though it is utterly and truely subjective since there is no reason the FBV Longboard can't do the job equally well if not better since it gives two foot pedals and a larger display and still provides every bit of the same functionality as the FBV Shortboard Mk II.  The next reply suggesting that the response was there to help Line 6 sell more equipment seems a propos.