May as well train a new desired pokemon with the same moves without special ribbons or mega stones. Especially if they are not in Omega Ruby/Alpha Sapphire before the national dex by default. Many pokemon are not listed even after transfer because they arent needed completion, so you'll need a way to get the mega stones all over.
But these 2 are sore thumbs among Mega Stones.
Lucario= Achieve all 5 Master Ranks in Contests and defeat Lisia (It's not worth the grind training a perfect Contest Pokemon, contests have a lot of the worst RNG involved.)
Garchomp= You need to capture 1000 opposing secret base flags. I doubt there are enough secret base players left active.