Specifically the ARAM brawl mode. I will list a tank, dps and support;
Tank - Stitches. Savour the flavour with double devour at 20… marvelous. (just finished a game with 50+ stacks of it, was bullying the enemy team back alone even when my healer was dead).
DPS - Valla. Pure Hatred build, never run out of mana, self heal and tons of damage.
Heal - Probably Deckard because you get so much value from his abilites.
Alarak. Infinitely stacking damage on a one lane map is busted on top of him being Alarak.
Stukov basicly a free win
Chromie basicly I never take big dmg and I can practice my Q
Kael brawl is one of those gamemodes where bombs are fun to watch
Chromie and Reworked Tass can dish out retarded amount of dmg.
Whitemane.
Teamfighting is easily the best part of wm and brawl is just that for 10+ minutes straight 
Malfurion is a close second for the same reason
For dps its hanzo. That eye watering poke and if we make it to level 20 you better believe your gonna be seeing those dragons every 10 seconds.
And then basically anyone with an infinite quest
Auriel. Constantly action means constant healing!