Support main (formerly Topgot main) here. If you are looking for someone to work on last hitting with, I recommend Varus. His auto attack animations are pretty smooth, so he works well for people new to minion farming. Also, you can take him as a reasonably effective AP mid for silly games if you have a light-hearted team!
If you mostly play support and jungle, you should have about three champs with different roles in both positions, so you can fill whatever gap is left in your team composition. For instance, I have
,
, and
in the jungle, so I can bring a tank, assassin, or fighter with varying amounts of utility. My solo-queue supports are
,
, and
. Most teams have some understanding of how to play alongside these three, so they are pretty safe picks. Take Soraka for sustain/poke if you already have a couple of front line fighters, Sona for strong in-lane poke and a AOE stun for team fight wombo-combos, and Leona if your team needs tankiness or CC, or if you want to run a very aggressive bot lane (perhaps you are laning with
or
).
Of course, I can't tell you who to main. Try as many champs as you can (ARAM is a great way to do this) until you find champs that you enjoy playing. If you like them enough, buy them, but I would not recommend buying champs until you are pretty well convinced you will like them. Don't be afraid to try some of the less popular champions. You might find that you like
as much as I do, or find that
,
, or
is unstoppable in your hands.
It's not necessary to "main" any champ in particular. Play what you enjoy playing, and you'll probably find just a few champs that you play more than any others because they are fun. Those are your mains, and you'll be a lot more likely to do well on a champ you enjoy than on whatever champ is OP in the meta of the time. As always, rule one is have fun, and help others have fun as much as you can. Play what you like and let others play what they like. Unless, of course, they like
.