Well, if by register all of them you mean all of them including Regigigas, to the best of my knowledge nothing happens. Well you do get bragging rights and the satisfaction you just completed the whole bowser and captured the hardest Pokemon in the game (Regigigas).
However if you mean all apart from Regigigas, capturing all of them allows you access regigigas whom you can then capture provided you have the 3 regis on your team - and then after you succeed there what I wrote about happens. xD
