Well, I would go with Reuniclus
It has better SP Attack, HP, and Defense, which is good because that means it can take hits well and deal damage well, rather than being a bit of a sweeper, like the other two, who are faster, but frailer. Reuniclus also has an incredible ability in Magic Gaurd, which protects it from all in-direct damage(recoil, weather, status, item held, etc.) Their typings are almost equal, with Gallade having one less weakness than the other two.(since Fairy didn't exist yet) However, Reuniclus is far easier to get, seeing as how you just have to train it to a certain level, which is about equal to the level in which you can get a dawn stone. It's first evolution is also available very early in-game, no inter-generation trading required. Reuniclus can also learn Rock Type moves to deal with it's Bug Weakness, and Dark moves to deal with it's Ghost Weakness, as well as learning Focus Blast and Rock Smash by TM to deal with it's last weakness in Dark. It also has really good lvl up movepool. It's second evolution is also better than Kirlia, which is the state these two will stay in for a while. It's growth Rate is also faster, making it easier to train. Also, I was just made aware you can only transfer between the generations in Post-Game, and that's another reason why Reuniclus is better.
So, if you want, an easy to get, hard hitting, surviving Pokemon, with an amazing ability, choose Reuniclus!
Sources: The typings page, and the pages for these three. Also Expierience.
