im trying to pick the last move for him and i want to know if earthquake or payback will be more useful on my gyarados. after a second of seeing this and without thinking i know most people will automatically say earthquake but if you take a closer look at payback, that move can be useful at times too. like say you predict those annoying ghosts switch ins like gengar who tries to use thunderbolt on gyarados and me knowing that, i hit them with payback on the switch in, then paybacks power will be doubled to 100 power and with the super effectiveness it will then become 200(its different than avalanche where you have to go last automatically or it will fail, payback works if you go last or if the opponent switches so i think thats the good thing)but earthquake can hit steels but most steels dont resist waterfall from a gyarados, and can kill fast electric types like jolteon with a dragon dance boost. both are very useful but which is actually better?
i would go with e quake as you said gengars ect can use thunderbolt but if you go for a speedy nature with speed evs e quake is the one for you plus there are not that many ghost sweepers like gengar out there and you could always switch to a ground type when they switch
but you also have to remember e quake does not effect gengar due to levitate but with the right nature evs ivs ect you should be able to out run a spatk natured gengar
as you also said when you send in gyarados use dragon dance to raise speed which meens you wont even need a speed nature
Actually, if you look at the Payback page on DB, Payback only gets doubled when you get attacked. So, definitely go with EQ b/c you have no need for Payback, most people don't use the Glass Cannon anymore, and besides, Waterfall/Aqua Tail can OHKO. Like boblylo said, just DD.