Sona is amazing. Build her:
Trinity Force
Mercs (or Tabi as necessary)
Frozen Heart
Wit's End
Etc.
She's IMHO one of the best tanky DPS on the map, being naturally ranged and yet naturally tanky thanks to her W and has the obscene burst in her Q+Power Chord+Sheen Auto-attack and the inherent slow + speed and great kiting. People go AP on her too easy; try the tanky DPS build.
Janna is amazing; standard AP. Catalyst + Fiendish Codex + CDR Boots into RoA + Cap, etc. Zilean works great with the exact same build. He's my pick for the strongest character on the map; the passive gives you a very opportune level-up in the opening fights allowing your team to use your ults before the enemy, and with max CDR + Rewind you can perma-speed two people. Your ult CD is ~30 seconds with Rewind spam. Your Bombs do AOE damage and you have ~600 base damage without items on level 9, which is very high up there beating basically all other casters (discounting ults of course). He also kites like a boss, makes Poppy pretty **** useless, defends points forever and is overall just obscenely strong.
Soraka I build tanky and go bot with. Odyn's, Frozen Heart, Mercs, Rylai, Abyssal into Deathcap, Sunfire Cape, Hextech Sweeper or similar. She beats most bots with Starcall spam and gives a steady stream of Wishes for the team. The MR reduction makes her effectively a Singed that keeps getting stronger as the fight goes on. Ridiculous. And the Silence so good too breaking spell comboes, doing decent harass, etc. I go Starcall > Astral Blessing > Infuse with one point in Infuse.
Alistar and Taric are obvious, no need for anything extensive on those.