I seem to find that Garen is stupidly OP. Personally I favour Amumu; he was fairly OP pre-nerf and quite strong post-nerf.
As Jungler, get Amumu (possibly Nunu)
As mid, get Zyra, Diana, Cass, Kennen, Galio
As top, get Kennen, Wu Kong, Garen
As support, get Sona, Galio (possibly Nunu)
As ADC, get Ezreal, Miss Fortune, Graves
Hope to god your team has AOE, and more importantly, AOE CC. 4 AOE Champs can win 4v5s., especially if it's something stupidly powerful like Galio Amumu Kennen.
Diana is especially potent in this regard, because she draws everyone in. Forcing the entire enemy team to endure a full Garen Spin or Kennen Ult or whatever AOE you have undoubtedly causes a lot of damage