What makes gear good?

While it's true that "good" gear is dependent on what you're specifically looking for, here are some good general tips to follow. 

Gear Enhancement: The higher the enhancement level, the larger the bonus to the primary and secondary attributes on the gear will be.

Legendary enhancement growth rate: 10%, 10%, 10%, 15%, 15%

Gear Level: The higher the level of the gear, the higher the attributes on the gear can be.

Gear Quality: The higher quality of the gear, the more bonus attributes it can have.

Necklace

Primary Stats: Hit Rate, Increased Damage

Secondary Stats: Armour, Life, Critical Hit

Ring

Primary Stats: Speed, Hit Rate, Increased Damage

Secondary Stats: Critical Hit, Critical Damage

Weapon

Primary Stats: Increased Damage, Hit Rate

Secondary Stats: Critical Hit, True Damage, Armor Breaker

Helmet/Chest-plate/Boots

Primary Stats: Damage Reduction, (Increased Armor / HP), Speed (for boots)

Secondary Stats: Miss, Resist

*If you're going for a glass cannon type composition, then you'll want as much speed as possible, making a double or triple speed stat on boots more important than even damage reduction. 

The Tower of Oblivion

At the higher levels of the tower you'll have to customize your squad for each specific level. You'll want to know exactly how much damage your damage dealers can take and still barely be alive enough to get one last round of attacks off. You'll have to know which enemies you can kill by swapping out your guys in order to do enough damage to take the needed targets out. All of these things are level dependent so you're going to have to analyze the level, your own heroes at your disposal, and calculate the best attack from that information.

How to build a strong team

Power Rating
Power rating is a little overrated. It doesn't reflect the "true" power of a squad / hero since it doesn't calculate damage reduction / increased damage. The power rating also doesn't know what the most important stats for your specific lineup is or calculate the synergy to giving %crit chance to a hero with a %crit damage passive over a hero with no %crit damage passive.

General Strategy
In general you want to stack specific stats on heroes that gain passive bonuses for that specific stat. For example, stacking life on a hero who has a +25% life passive is 25% more efficient then stacking armor on the same hero.

Defense
For defense you want to prioritize damage reduction first, then armor and HP afterwards. Focusing on % miss gets harder as mobs and players focus on % hit more.

Armor vs Health Points
You want to keep a good hp to armor ratio in order for your defense to be most effective (mathematically). I find that a 10 HP to 1 armor ratio is a good general rule to follow.

Offense
Speed us probably the most important stat in PVP combat. Most people have a very squishy backline that deals a lot of AOE damage so as long as your backline gets to attack first then you can kill the enemy backliners before they ever attack. Also, even if both your backlines survive the first round onslaught, the same principle still applies for the second round.

*Note: For very tanky team compositions, speed is less important as the number of rounds in battle increase.


Critical Chance vs Critical Damage
You want to keep a 10 Critical Damage to 1 Critical Chance ratio in order to optimize your damage output.