As far as pub matches go, double support (Lucio/Mercy) is more likely to win than most stuff you can pull out. Squishy comps are Widowmaker food, tanky comps are melted by a half decent Reaper. Turret spam takes a team effort of Widow/Tracer and friends to break past, and its a real clown fiesta if they have a Mercy (which they likely will).
Personally I'm almost level 50 and mainly play Reaper, of which i'm in the top 40 across the EU server (was a surprise to see) and 60ish on NA, although that number will likely drop every time I sleep. I'd say hes a viable pick in any match that requires an offensive hero, but finds his best games on defense, as you can teleport to a flanking spot to ult the backline, kill their support and also melt their tanks. He can also do a similar job to McCree as an anti-flanker, but getting a decent Tracer to stay and fight you isn't that easy whereas McCree has his flashbang to force her to sit. Most Tracers will take the fight (and lose) but I think they are slowly learning. If they blink past you then you can chase somewhat well on most maps and pressure her to recall back, keeping the supports/turrets safe. Decent pick on control point (king of the hill) maps too. He has no bad maps due to how he can force fights on his own grounds and escape, but has some more annoying matchups mainly vs turret builders and McCree if you don't dodge the flashbang with wraith form (v difficult to predict, ping can screw you). Zarya is harder to kill than other tanks but still very doable. Mei is a pain in the ass if your aim is off and wraith form is on cooldown.
I'm gonna drop my thoughts and opinions on the maps rather than the heroes as I believe everyone's opinions on heroes mainly form from playing against a certain dude as (your main) with the exception of McCree who most agree he could do with a small nerf. Maps are easier to discuss as most have used different heroes and comps on them. I can drop my opinions on the other heroes later on.
Dorado (Payload): Pretty well balanced. Every single hero has potential to get shit done on this map, and each payload checkpoint is equally defendable and pushable on offense.
Hanamura (Assault): This map is dog shit. Symmetra and a turret dude is near necessary on defending point A (or at least run the timer down so you can easily hold B), and you can go full cancer (Mercy/Torb/Bastion/Reindhart/McRee/Widow) on point B and choke the fun out of the enemy team and be extremely hard to push. If you are on offense you have to snowball from A>B before they set up properly (A is easy to take with flankers and some effort). If they have dug in, play 2 Widows and push 1/2 other dudes down the upper right balcony to pressure the turrets. If you are <Lv40 and playing solo (therefore teammates do not fully understand counterpicking/changing hero) then you just gotta hope for the best. This map is all over the place.
Hollywood (Hybrid): Taking A can be a real pain in the ass vs a competent defense, and the open area just after A can also be hard to push. I'd say this map is more reliant on skill/strategy to win than the others - if your offense sucks, you lose. If your defense sucks, you lose. Sounds really obvious but it is far more noticeable on Hollywood, so I guess that makes it balanced.
King's Row (Hybrid): Seems to require Symmetra or competent Mercy to have a chance of holding A for a period of time. If you lose 4-5 members at once, A is simply gone. Run down the timer on A then hold the huge ass chokepoint (inside the factory) at the last 2 payload checkpoints for the win. Beware of Widowmaker + flankers here.
Illios | Lijiang Tower | Nepal (King of the Hill): Lemme preface this by saying I'm really not a fan of koth maps. The picks are incredibly stale in pubs (Lucio/Junkrat/Mei) and it seems the pro meta just takes this to the extreme, so I'm glad I don't currently watch it. If you don't deny overtime you risk a pretty damn good chance of going from 99-0 to losing. People who pick snipers on king of the hill maps tilt the living shit out of me. Unless there is a Pharah going off and it's a counterpick, pls dont do this. Lijiang Tower is far better than the other two in my experience. The other two you can just pick dudes with knockback (read: lucio) + tanks and hold the point.
Numbani (Hybrid): The first point is usually easy to take just by walking at them unless they have turret spam (basically Torb) + Widowmaker. For whatever reason it's a rare sight, so A is free. Mileage may vary. The payload is not difficult to push until you get through that underpass area near their base. Then its fair game. Get flankers to take out their sniper + support on offense, and on defense, Reaper is a really good pick in my opinion. Besides that its just standard stuff - momentum is key.
Route 66 (Payload): I like this map. Many games I've both won and lost at the first checkpoint, and it feels balanced the entire way through. Bastion on offense with a Reinhardt is surprisingly effective.
Temple of Anubis (Assault): Chokepoint central. Communication, good team comp and also good ultimate use wins this for either side. There's a good amount of amazing flank spots - learn
this spot (works from both sides, it comes in handy if you play flankers)
Volskaya Industries (Assault): Snowball from A>B cuz if not you are in for a grind until the timer runs out. Offense on A is 55/45, B can be a nightmare without amazing teamwork. Flanking is your friend, as is Reinhardt. If there is a competent Mei on the defending team then prepare for misery.
Watchpoint: Gibraltar (Payload): One of the more annoying payload maps. A sniper on both sides is actually ok here, as flank routes are limited to like 1 hallway on each side and they barely reach anywhere good, and on offense you can take out the turrets/enemy sniper. Soldier 76 has a field day here. Reaper is more useful than you think if you play it smart, as he can teleport between the staircases out of the line of sight and really pressure the backline on both offense and defense. The last payload checkpoint takes some real effort to take.
One tip for everyone: focus the damn Mercy before using your ult on the priority targets. This goes double if you play the offensive heroes, although Tracer is less reliant on it as bombing one person is hard for Mercy to revive and call it worth.
This game is great.