In my opinion:

Anivia (Mid)
Sona+Corki
Kennen (sololane)
Rammus (jungle)

Dream team of 5.

Maokai
Galio
Kennen
Nocturne
Anivia

Galio = Magic defence tank. Ultimate to keep everyone in place whilst maokai ultimate and releases his damage upon galio's ultimate ending and kennen using his ulti whilst all are focusing galio during his ulti. Nocturne and be AD and anivia can if needed, also use her ulti whilst all the other ulti's are going off.

Still a pretty nice team composition. You have 2 solid tanks, one of which can deal quite a lot of damage and the other has a nice root. Theres a lot of potential in the team.

I might swap out maokai for a nasus if they were heavy of those tankish people (galio taunt + spirit fire would be handy).

Taric, Sivir, Amumu, Anivia and Jarvan.

Amumu: jungle, great stun and root, good AoE
Taric: botlane with sivir, great in duo lane, ulti to buff jarvan and sivir.
Sivir: botlane with taric, great burst damage with boomerang blade and lots of AoE, ulti to help chase, escape and kill.
Jarvan: solo top lane, great off-tank with good CC and damage, additional attack speed for sivir.
Anivia: solo mid, high damage output, great AoE.

Teams will melt in seconds.

Gragas - top AP build
Amunu - tank, jungler
Corki - bot
Nidalee - bot
Malzahar - mid

In my opinion a Dreamteam does not exist of certain Champions picked,

but more of people that :

play together as a real team,
have fun,
don´t flame or react on flame,
and don´t care to much of winning or losing.

Play who you like.

Have fun all,
Az.

Brand mid
Ashe+Sona bottom lane
Amumu jungler and tank
Renekton off tank solo top.

Irelia solo top
Orianna mid
Vayne bot
Janna bot
Amumu jungle

Me My 3 mates and one of their girlfriends...Oh We are talking about champions?