A lot of people don't play Galio because of the assumption that he is only effective against double AP comps, because of his passive. I've always treated his passive as a bonus to his kit, not a condition to fill. If I'm against a double AP comp, yay me, because bonus AP from all the MR I'll be building. If not, I build Athene's, then some armor.
Full damage Galio is rather terrifying, especially since they just buffed the damage on his ult (max damage of 720, scales up to 108% of your ap if you taunt for the full duration). You poke with your Q's and ult only after your tank has initiated. Glass cannon Galio doesn't really work if you're the only tank on your team, or if there are several enemies that can interrupt your ult.
For support, I prioritize Face of the Mountain, Mikael's Crucible, Sightstone, and Boots of Mobility as my core items (I always aim for 40% CD, too). I don't get Talisman or Righteous Glory because I tend to hang back by my ADC rather than initiating team fights. I max Q first, then W, because Bulwark is so beautiful when you're laning with someone. My job is to wittle down the enemy with my Q's (since the base damage is rather high, 300 at max rank), then flash ult for the kill. The enemy duo and their jungler/roaming mid laner will be hesitant to dive you under your tower because of your ult. You can stay in lane a long time from the sustain from Relic Shield + tanking minions while Bulwark is up.
My favorite ADC's are probably Ashe and Kalista, because of their CC. Sivir and Varus would also make landing Galio's ult easier, though I don't lane with them often. Laning against a Kalista and a Lucian makes it hard to land your poke, and Braum and Janna can knock you out of your ult. Even though you dont' deal damage as Support, the slow from your Q and the speedup from your E can allow you to peel for your team or chase someone down.