Worst ult imo is Tracer, Roadhog, or Soldier 76. Tracer's is really only good for reliably killing one person at best, has crappy range and radius, and doesn't even one shot tanks like Reinhardt or Roadhog (as far as I can tell, anyway). I mean, it's nice that it's basically a free kill but it's just not that impactful or helpful to the team in the way other damage dealing characters do.
Roadhog's just feels weak. It doesn't really do any more damage at range than normal shots and since it constantly pushes people back you'll find yourself doing relatively little damage with it unless you can pin someone up against a wall. It also jerks his aim all around making it difficult to use and removes his two far more useful normal abilities for the full duration. It's one of the only abilities in the game I find myself deliberately avoiding outside of very niche situations.
Soldier's isn't bad I guess but it doesn't really give him any more killing potential. It's just a crutch to help his aim (and iirc doesn't even apply to his rockets). You can accomplish the same effect, full time, by just being good at the game.
Best Ults are probably Mei and Zarya (almost guaranteed to wipe a team if properly coordinated), Hanzo (perfect area denial, forces the enemy to vacate an area or die at no risk to Hanzo himself), or Lucio (basically renders your team invincible for a few seconds after it goes off, without removing Lucio from the fight like Zenyatta's).