Of those two, definitely Alistar. Taric is cool and all, but Alistar is much more useful in general, gotta be careful not to shoot your opponent out of your ADC's poke tho
. Though you should probably be going for Leona/Braum/Blitz first, as they're overall better (and Thresh is best, but he's pretty tough to play) Leona is /the/ tank support in terms of sheer tankiness, and she's got some good CC. Braum is also super tanky, and is good with an ADC that has long range AAs (Cait, Jinx, etc) And Blitz is Blitz with his Q grabbing the world
Tank supports are generally harder to play than offensive supports, so no matter who you pick there'll be a learning curve. If you just want someone simple but strong I'd say go for Leona, because you literally just dash in, drop CC everywhere, and then body block skillshots. Definitely not Alistar or Taric. Alistar takes a lot of positioning to play well, and Taric needs a good team comp to perform adequately (though in the right team he can be quite the terror)
Oh, and offensive can also mean a support that goes after the enemy team instead of trying to cover and peel for their ADC, so you weren't entirely wrong. And by that definition, Janna would be a defensive support. But I'm more speaking in terms of tank vs. damage output, especially since the former is a bit of a loose definition and generally comes down to how a champion is played, rather than their kit
Hey there, I'm Alexus, call me Kosher if that's too confusing for you ^^