Salamence, one of Gen III's Pseudo-Legendaries sometimes called Dragonite's Expy. Recently sent to the Uber tier by Smogon but whether or not it will stay there is yet to be seen.
Hydreigon, Gen V's Pseudo-Legendary and the only Special-oriented one. Its also the only Dragon Pseudo Legendary to not have double weakness to ice.
Who do you like more?
Who would win in a battle?
Who is more useful.
Note that there is no way of knowing the answer to the last 2 questions but I put them there just for the sake of tradition.
Personally, I like Salamence much more. It looks so much cooler than Hydreigon. It also has Dragon Dance which Hydreigon doesn't have, IIRC. Hydreigon looks too wierd for me.