League-of-legends – Is Sona a better champion as defensive support or an aggressive one

Malzahar is a perfectly acceptable AP carry. He has strong burst, good damage scaling and good teamfight presence.

There are a few reasons why you see him less in tournaments:

A) believe it or not, people got bored of him. Malzahar was a high level pick/ban for a long time, and after his nerf on W, people found different champions to use despite him still being viable

B) QSS can REALLY hurt his ability to fully combo high priority targets

C) No escapes. He's extremely susceptible to ganks. Most picked AP carries nowadays are a little harder to tag down. Think of Kennen, Ahri, Ryze, Morg etc. They all have good ways to ward off ganks. Malz does not

Don't let this sway you from playing and mastering him if you love his playstyle. He's still an extremely potent champion

The job of support champions is to make the other champions look good. They can do this in many ways: healing them (soraka, sona, taric, alistar) and Buffing them (Janna with her shield, Nunu with Blood Boil). The crowd control spells help in the team as well which can help in making a support better. For example: Alistar has a weak heal, but the CC he provides makes him a good choice.

So essentially its the buffing and healing that makes a support, but the CC they can provide can make the champion more appealing.