Both have their ups and downs, I've played both and honestly I'd have to say Viktor if you want to have fun stomping people who don't think he deals damage, I remember a game awhile ago where I was Vik against yasuo, for 20 minutes I was 1v1ing the yasuo with no problems to the point where the jungler and support had to camp my lane, my first kill against Yasuo had him say "WTF YOU DEALS DAMAGE?" which just made my day so much. Viktor deals a billion damage as long as you're able to consistently hit your E, but after awhile that becomes second nature, people underestimate him and lose games for doing so.
Galio on the other hand is also insanely fun when played as support, just rush
and
pop both actives while chasing, get 600 movespeed, walk into the enemy team and ult, no one expects the insane speed those 2 items give you so you can easily get in and ult people, feels so satisfying too, but his early game feels really weak until you get your ult, late game engages though will win you the game.
It all depends on what you want to do, do you want to dominate the lane and become a team fighting monster who requires good positioning to not get killed? then go Viktor. Do you want to play safe early game and win the game for your team later on by flash ulting into the enemy team and laughing as they all die around you? then Galio.