I simply use the Blissey secret bases on Omega Ruby! They are by far the best leveling method, considering they each give ~20K experience points with Lucky Egg, which becomes double after a Lv.3 Exp. O-Power for an amazing 40K per Blissey! Also, they are Triple Battles, meaning you can level up lots of Pokemon at the same time! After just one battle, most Lv.1 Pokemon will just jump to around Lv.45, which is insanely fast! Since there are 12 secret bases like that, each with 3 Blisseys, you normally can get up to 1440000 experience points a day (that's right, more than a MILLION experience points!), more than enough to reach Lv.100 with everything besides the Pokemon belonging to the Fluctuating experience group! However, if after battling them you simply add them as your secret base pals, you can just battle them once more each, meaning DOUBLE that already insane daily amount of experience! And don't forget, since it's Triple Battles, THREE Pokemon will get ALL that experience each, while the rest will still get half of it, but you can just switch them in so your entire team gets the entirety of that experience! Finally, all that unbelievable amount stated above is simply what you will get after only a Lucky Egg coupled with the Lv.3 Exp. O-Power, so if you got something from a trade or have stopped a Pokemon from evolving and it's beyond the level it evolves, add a multiplier of 1.2 for each of these cases, making the total possible DAILY experience a godly amount of 2073000 experience points WITHOUT ADDING THEM TO YOUR SECRET BASE, which would become 4 MILLION PER DAY if you decide to add them!
