I'm level 90, and I have pretty much all of the best heroes at level 90 and max Orange. In the past seven days, I've finished #1 5 of those days(I have a very quiet time zone, this is the biggest reason). Here are my thoughts on heroes:
The Cookie Cutter team
The main defensive team is Admiral/Silencer +3 mixed magic/physical. Most often and most effective, this is Admiral/FD/Silencer/Machinist/Mystic. I see a lot of variations but these are usually easier to counter. Both counters I'm familiar with use unusual heroes. The easier counter is VW/EB/CW/VS/Phoenix. A lot of people have the first four but not Phoenix, but you need Phoenix to kill FD. OC can be a substitute if you don't have Phoenix. This team is sketchy versus the version with FD/Machinist/Mystic, but tends to be much better against other versions of this team. The best counter against the cookie cutter build is Admiral/AS/LG/HN/Commando. LG and HN are not heroes many people have access to. This combo can almost always kill Machinist before his ulti. Commando pushes the back row damage. Hidden Needle is your finisher and with her shield up she will beat FD.
Tanks
Admiral--she's head and shoulders the main tank right now. The most powerful teams are mixed magic/physical offensive teams and she is the best tank for those by a mile. The more offensive tanks don't really last very long versus Machinist's ultimate or a transformed FD, and these are everywhere. The cookie cutter team right now is Legendary Admiral, FD, Silencer, Machinist, and Legendary Mystic. This team is very difficult to counter, the primary one uses two heroes very few people have.
Warchief--with the emergence of mixed Admiral teams, Warchief kind of seems second best at everything. He's probably the best tank for a pure physical offensive team, but those teams really aren't that strong right now. He's got some decent durability and his life steal aura matters, and he deals good damage. He just doesn't have a team.
Drunken Master--Panda is painfully underrated right now. He isn't dominant like he used to be and people have discarded him, but that is IMHO premature. He's still got 40k-ish HP and crowd control, as well as dealing good damage. I find myself using Panda a lot on offensive crowd control teams, like Panda DV VS Shadowleaf plus FD/CW/AS/Silencer, which counter some GA teams. Against a team built around Ember Blade and not a lot of magic, Panda is my go-to tank.
Vanguard Warrior--not a tank, but the front line for very common and powerful offensive teams. His delayed appearance means your mid liners will likely ulti first and he clumps enemies into a pile. Most popular is VW/EB/Silencer plus two Mages, but that loses to mixed Admiral teams so people have been experimenting with a variety of setups.
Depths Voice--I love her legendary, but I've come to the conclusion that she can't do it alone. She could probably solo tank a pure physical team but I rarely see those and they can be beat by most anything anyway. She also stands behind AS and FD and can't solo tank for them. I like her on an offensive team with a lot of CC and Panda/WC/Admiral out front.
Iron Hoof--he's mostly eclipsed by Admiral, as he doesn't compare with her on offense and he's not as good tanking for a sturdy team. What he is good for is that he's better than Admiral for a 4 mage team or a very defensively challenged team as Admiral doesn't have that shield that takes damage off the squishies.
Psychopath--He has a very narrow use in that he takes magic damage well and is an offensive force. No other tank who can take magic damage well can match his offense, and he can counter some setups given this.
Death Knight--has a very narrow use where DK/AS/DM is very strong in GA against Admiral/IH + 4 Mages.
DPS/Support
Shadowleaf--she isn't top tier anymore but I still use her, more for her silence than anything. She deals medium damage, buffs physical, and has an area silence. I use her when I need all 3.
Emberstar--She's easy to build so a lot of people have her at full Legendary and I've seen her somewhat often. She never really seems decisive, she's not as good as Machinist. Her ultimate has a nasty habit of hitting clones.
Machinist--The premier magic DPS, his ultimate shreds the front line and clones are little defense against him. He's not as good as the best physical DPS(FD/EB) because he isnt nearly as survivable. He is also a little stronger than most magic heroes against AS since his laser attack almost always hits AS right in the face.
Vengence Spirit--A mainstay and powerhouse. She deals major area damage and has very strong crowd control which she uses more often thanks to her energy drain. She's also very beefy for a midline.
Silencer--he's the glue that holds many of the strongest teams together, and is strong on physical, magical, and mixed teams. He literally belongs on most any team. He doesn't look that strong on paper but most teams are better when he's on them. Very underrated damage dealer, very survivable, and very disruptive. His damage isnt subject to dodge.
Lunar Guardian--after being second rate for a long time at level 90 she comes back in a big way. The most dangerous teams involve squishy Mages in the back, often behind Admiral. The best way to beat these teams is to bypass the tank melt the back line and LG has an area auto-Attack and a team buff and can push back row damage to the point where it can make a difference.
Frost Mage--she's generally outclassed by Mystic, but the 4 mage team generally uses both and she has some nice synergy with Vanguard Warrior as he clumps the enemies into a pile she can destroy.
Old Curse--these guys are still a staple. Alone among the big magic heroes they have crowd control, and it's rare OC doesn't top the damage meter. Has a nasty tendency to charge enemy energy, but great area damage, crowd control, and being one of the most survivable mages keeps OC in the rotation.
Mystic--he's like Silencer in that he just has that indescribable something that just plain works though you can't explain how. He buffs the other Mages, makes physical enemies miss, and his ultimate is scary if slow. Nothing he does is really that special but he's more than the sum of his parts. If I'm running two Mages, one of them is almost always Mystic.
Death Mage--I find her useful on attack in two specific circumstances. Against the VW/EB team, the strongest counter is DM/OC/FM/Machinist/Commando. The other time is when I need the team silence, I need it faster than Silencer can deliver it, and Shadowleaf doesn't fit the team. She isn't going to survive to finish her ulti, but she'll deal decent damage and silence before she dies, and that's all I expect of her.
Arcane Sapper--he's never the main focus of the team, but he's still very effective support. Combined with Commando and Lunar Guardian, he can usually kill Machinist before he Ultis but he needs that much help to do it. He also is a good offtank and magic counter.
Fallen Dominion--one of the premier DPS out there. Very high single target DPS and very survivable thanks to life steal. No tank can last long against transformed FD, and he can win the end game outnumbered 3-1. CW clones and magic damage are effective but don't entirely counter FD. He is weak versus the VW/EB teams.
Hidden Needle--she is the most effective weapon against cookie cutter mixed Admiral teams and can be effective on a VW team. She needs a lot of support, and Arcane Sapper. Arcane Sapper can off tank and keep area damage away from her. She's very squishy and her ulti is slow to charge. Her ulti hits for 20k, half of that area and if she can get a kill she puts up a shield, is almost invincible, and charges her ulti Lightning fast. Her basic attack doesn't miss and hits the entire team for 3k. She can get her shield from killing clones. She is the only hero I've seen that can take down transformed FD 1 on 1. Admiral AS LG HN Commando is the team I use most often.
Phoenix--Phoenix is the best area magic DPS, better than OC because Phoenix deals damage in one big burst. The downside is that Phoenix is very easily disrupted, avoid CC and AS. The best Phoenix teams have a backup plan, as he's unreliable. He's strong enough to be worth the risk.
Cloud Walker--rarely deals big damage, and has a lot of counters. That being said he's great support in the offtank role. Avoid Ember Blade at all costs, as CW is a downright liability there.
Ember Blade--the other premier physical DPS. His ultimate helps keep him alive and he's annoying to finish off. He's an area damage dealer compared to FD. He's difficult to counter, he's squishy overall but crowd control can block the skills he uses to stay alive, but he has a nasty habit of avoiding death. The best way to counter EB is to ignore him and deal with the rest of the team.
Commando--isn't the requirement she used to be, but still gets use when countering with a fast offensive team that needs the early push.
Other heroes I don't have or haven't seen enough to have a strong opinion.