Choose the roles you play mainly and then choose from there.
Me for example: I play Mid, Jungle and top.
With those 3 roles I like to have at least 3 main champions for each. 1 Main champ, one secondary just incase and one third incase for some reason my champs are banned/taken (although not likely to hit number 3)
Mid: Vel'koz, Orianna, Viktor
Jungle: Olaf, Nocturne, Shyvanna
Top: Olaf, Pantheon, Nocturne
It all depends on your preference. You can always switch around to have fun but if you really wanna choose something to stick with, look at the champion itself and their backstory. Maybe you can love champions for their storyline and who they are - making them more interesting. I got a friend who plays Renekton who became unoffically the top played Renekton (split between 2 accounts. One was ranked 3rd (not many games from rank 1 and 2) and his second one was rank 5 or 6). He plays Renekton so much because he loves his concept and his story. He loves how he is this slaughter machine that looks like he came from the depths of another strange world.
I love Nocturne for example because of how he looks. He is this badass unknown entity that has a mysterious background. There isn't much about him and his kind and if you read his story - he simply just 'appeared'. I'm sure there is more Lore out there from Riot but here is what it says:
"Before Nocturne, people believed that dreams were figments of their imagination, meaningless images that flashed through the mind when one slept. This belief was put to the test when a rash of sleep-related incidents started afflicting summoners of the League. Some would wake up screaming, terrified and beyond consolation. Some could not fall asleep, slowly going mad as the nights ticked by. Some simply never woke up. Physicians were baffled until a Field Architect happened to pass out next to a nexus on Twisted Treeline. Witnesses said he cried out once and then stopped breathing. Immediately after, magical energy arced out from the nexus, and Nocturne appeared.
Nocturne did not take his introduction to this world kindly. He slaughtered everything he could find before summoners were able to magically confine him. After a period of intense study, League experts divined that Nocturne hunted summoners in their sleep, attacking them in a place where their magic was useless. This seemed to be his only purpose. The families of the victims demanded justice, but League officials were concerned that death might only return Nocturne to the place from which he came. They bound him to a nexus fragment, trapping him in the physical world. As punishment for his crimes, they allowed summoners to call upon Nocturne in the League matches, bending his will to the summoners he hates and creating his own personal nightmare. League scholars don't know whether he truly came from the plane of dreams, or whether there are any more like him. Some theorize that the summoning act affected summoners' subconscious minds, luring Nocturne to them in their sleep. Perhaps the most disturbing theory is that Nocturne is a person's nightmare come to life. If this is true, they wonder, who is the dreamer?
''The darkness is closing in...it's pitch black now...but I can still see him...''
- Kelvin Ma, patient #4236"
That alone draws me to nocturne. The same goes for Pantheon and vel'koz. Maybe you like the cute things or bad-ass type girl champions that destroy other champions. The choice is yours but if it helps, try to get into a more personal level with the champion to get your 'main' or 'mains'!
AD Carry: 
AP Carry: 
Also the fewer champs you play, the better that you will become with them and the easier new champs will be to learn.
(you should never play singed regardless) 
etc.
will always have insanely high pick/ban rates. There really is no such thing as "one tricking" these champs since they will be picked or banned away from you a majority of the time.
etc. These champs are hard to play consistently since you have to readjust yourself to their new numbers every other patch. One of the biggest advantages to being a OTP is knowing how much dmg you can take/deal to enemies. OTP's are almost never surprised that they won a fight because they understand the limits of their champ so well.
and any sup that isn't 
or 
