Use Gyarados with waterfall or golem against Ho-oh?

Gyarados with WF/HP will not get 1HKOed by Solar Beam.

I love how Gary is relevant again!

Against solar beam: Gyarados with waterfall/hydropump
Against all other moves: Golem with double rock

Gyarados is a good option against Solar Beam. Overall Raikou is the best, Zapdos second best and Gary comes in pretty much right after the legendaries. Best DPS is an endless stream of RB Golems or below level 28 SE Golems, but that's pretty unrealistic for most people.

Against FB and BB Golem is better by a mile and a half.

if solar beam gyara/zapdos
for other moves, depends on weather, if raining gyarados and zapdos/raikou are probably better, otherwise golem will do the job