Ork on a Bike is only cool if you plan to have a lot of Bikes. If you DON'T have a lot of bikes (at least 1 big unit) he's not worth it.
If you DO have lots of Bikes (any kind) get this badboy a powerklaw and you are all set.
The Big Mek is the obvious choice - you can attach him to most any unit and give him kool stuff like a KFF, Toolz, etc.
On second thought, you can have TWO HQs, so why not one of each?
Sol