I think the fact that mercy and lucio are must picks at the moment has less to do with their relative strength so much as a lack of viable alternatives. I think this is a problem that will smooth itself out as blizzard adds more and more viable healers to the game; hopefully that is a large focus for them in their first few hero expansions.
From what I hear this is a problem almost exclusively on consoles, but torbjorn is too good, at least for quick play. A single torbjorn, even a well played one, isn't actually much of a problem, but when another torbjorn sets up a turret to cover flanker angles or a reinhardt sets up in front of him to cover, it becomes a much tricker proposition. Even then a well organized team can force through it, but it only takes 2-3 people on defense to set up what essentially requires a full 6 man coordinated offensive to bust through. That is incredibly rare in quick play. I'm not sure how you go about balancing this as I hear torbjorn isn't a problem on pc and is essentially a joke in the tournament scene. Perhaps with the coming of competitive on the horizon the issue will become less severe, but I have my doubts.
Torbjorn is only a problem on console yeah, he's not viable on PC lol and in quick play he's only ok at specific maps on defend, and he has hard counters that can just shut him down easily. He has a huge problem in the fact that if his turret is down and he dies along with it, then it's really hard to get back up if the other team is pressuring u well, not to mention so much can just kill it. If you have a Reinhardt in front of you literally anything that can shoot can take it down, mercy's pistols, widowmaker, etc.
As for mercy, i believe she's a must pick not because there isn't an alternative, but because her ult is ridiculously good and charges super fast, pretty sure it's the fastest charging ult in the game. With the metagame going triple tank these days as well she can heal them and like get an ult 15 secs after having already used one lol.
I think Lucio is fine because his ult takes a long time to charge relatively, even if it can be just as game-changing. He IS probably like the best or at worst a top 3 hero, but I don't think it's because he's broken/too good, he's just a really good hero.
tbh I would like to see McCree's damage falloff lessened a bit; you really need to have extreme precision to hit anyone at a distance, especially with the FUN 20 tick rate mandated on the servers. 76 completely outclasses him at precision shooting long range and destroying turrets & pharrah. Only thing mcree does any better now that FtH is nerfed (to a good spot) is stun tracer, and honestly tons of characters control tracer as well (roadhot, zarya, pharrah)
Yea McCree has gone from an insta-pick hero to a very niche one that only works on certain modes/maps/as a counter to a few heroes. Too harsh of a nerf imo ;_; he doesn't need all that fall off damage :(