Sona is amazing. Build her:
Mercs (or Tabi as necessary)
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.