Depends on your Elo in all honesty and what Elo you'll get paired with. If you're low elo, the OP katarina and master yi are always going to be viable as nobody knows how to CC correctly.
Other fun viable picks are:
Fiddlesticks - massive early game poke with crows + ult and fears work so well in teamfights with zhonyas.
Nidalee - Shes a monster with poking. Spears do so much early and she has so much mobility.
Amumu - 5 man ult? It's over. Enemies can pack their bags and surrender. Not to mention you're a tank.
Garen - Sustain passive, can't back in ARAM, so why not passively heal to full in 20 seconds?
Annie - Multiple target Tibbers with stun. Can easy deal LARGE amounts of damage fast.
Blitzcrank, Darius, Thresh - All can pull in champions and can change a game in a matter of seconds.
Jinx - Her W? Easy poke early. Her E? CC lock multiple people. Her Q? Does massive AOE damage late game. Her R? AOE damage and snipes.
Lux - Her ult cooldowns when at max level with 25% CDR = 37.5 second. Her combo does well over 1k damage easily and with the low cooldowns, she's a troublemaker in a teamfight.
Malphite - Ult can initiate and do decent damage meanwhile enemies will focus his tankyness.
Miss Fortune - Ult is AOE and deals massive damage.
Ryze - AOE ult is amazing and with spell vamp, you literally won't die if you build a tanky mana ryze.
Sejuani - Ult can freeze everyone.
Shaco - AP shaco, the boxes do damage, and so does he.
Sona / soraka - Heals and can do good damage.
Veigar - AOE stun - Aoe W, lots of damage. Can nuke somebody.
Xerath - long range poke for days.
Zilean - Spam W and get R's every 45 seconds. Q W Q bomb combo lvl 3 and do good damage.
All the other adcs - massive damage dealers.
Have fun!
