Honestly, Octane is currently my favorite legend, since I solo queue pubs and I like to go for high kill games. I did get to Diamond with Octane in raked as well, though. The most useless legend for me is Wattson. She's great for people playing scrims and tournaments, but her abilities are quite useless in solo pubs.
What I like about Octane is that he is the fastest legend in the game. Sure, other legends have some movement options as well, but Octane can keep spamming the stim button to outrun and outmaneuver (by constantly repositioning quickly) everyone else. In ranked, this meant that I could escape teams of sweats who were determined to hunt me down across the map after instantly wiping my lower ranked random teammates. In pubs, this means that I get to ongoing fights faster, so I wouldn't miss the 3rd party opportunities, and it also means that I can quickly get away from bad fights or reposition myself from the low ground spot to a god spot on the high ground (this is where the buffed launch pad is often very useful). The fast movement also means that if I drop near enemies, I can reach the good loot first and possibly turn around to find unarmed targets to shoot at. Octane can rapidly cover large territories to pick out the best loot.
As for bringing value to the team, it depends on how you use it. When your teammates like to run in separate directions on their own then Octane can rapidly catch up to them when they run into enemies. If the teammate is down, you can throw him the launch pad, so he can jump to a safe location where he can be revived. If the teammate has been eliminated then Octane is one of the few legends who could pull off a heroic banner rescue from under the enemy's nose (sure, Crypto drone is the safer option, but I still prefer to have a high mobility legend in the game). The launch pad is also great for launching your teammates onto various rooftop spots where you can take some free shots at the enemy and indoors you can put the launch pad in front of the door to make any surprise attacks more difficult. Also, you can throw the launch pad under a door to launch off an enemy who is blocking it on the other side :D
The only thing that is an issue for me with Octane is that he doesn't exactly have the smallest hitbox, so more bullets will hit you, but then again he has automatic healing, so you can heal while running or fixing armor and it's also good when you're out of syringes (that came in handy when ratting out in ranked as well, especially since the automatic healing is silent).
If I see my teammates picking Octane, I'm thinking "good, at least he has options to get out of bad spots." If they pick someone else like Lifeline or Caustic for example then what will they do when we inevitably get third partied (evidently this game is all about 3rd partying) ? If a full squad ambushes one of those legends then what will they do, other than scream for help before getting instantly wiped?