Ok, leaving bias out for Masquerain being my favorite Pokemon, but I think Masquerain is by far the better one. Its base stats don't hold up near the end of the game and its level up movepool and typing leaves much to be desired, its TM movepool is surprisingly wide, and DexNav gets Surskit access to some very nice moves early on, namely Signal Beam, Psybeam, Mud Shot, and even Hydro Pump. Feebas also gets some nice moves like Hypnosis, Brine, and Dragon Pulse, and Milotic is really good ingame and I shouldn't even need to explain why. The problem? You can catch a Surskit in Route 102, but you can't catch a Feebas until Route 119. By then, you'd already have a Surf user. Evolving it is also a pain since Prism Scales are unobtainable in ORAS, though maxing out Feebas' beauty stat isn't as painful as Generations 3 and 4. Milotic just comes too late to be worth the effort when better Water types were accessible much later. Surskit isn't even the best option for a team member, and it doesn't even get access to Surf, but It just comes so early compared to Milotic.
Don't get the wrong idea though, Milotic is awesome, and Masquerain is a very solid pick ingame.