In Rise of Civilization (RoC) there are many heroes to choose from, the question is the best one?
A good hero is a strong hero. Everything in PvP scales with Commander Power. Of course different factors play a role: Troop size, Talents, Skills and Buffs. Some factors are fixed by the game or your progression, like Buffs and troop size (for the most part), so we'll focus on what you can influence. This begins with your choice of hero.
Like Food, Wood, Stone and Gold, experience and sculptures are a resource too and have to be gathered. You'll get an advantage early on in the game, if you invest in the right hero from the beginning.
Here's my first consideration: Legendaries are the strongest, but not the best heroes. The amount of experience required to maximise a legendary is huge! In experience and statues. The big problem here are not necessarily the experience point as you'll over time acquire enough to get him to level 60, but the sculptures and Stars required to do so. And they're rare if you don't spend the money. And even if you spend big money, you can only ensure to maximise those heroes with the mightiest governors. (Side note for the big players: You need to score first place 3 times and second once or you'll lack sculptures to maximise your hero. If you can check those requirements go for Ceasar! One of the most versatile legendary heroes.)
So an epic hero should be chosen. This hero can't be a specialist for one unit type, because you can build a mixed army faster. A single unit type army is for the endgame. He shouldn't be a defender or a support hero. This leaves the choice between "Scipio Africanus" and "Osman I". Both are solid, but Osman I is better.
First of all, Osmans overall commander strenght is higher than Scpios (Osman I: 88,300 compares to Scipio: 80,300). Although the margin is "only" 8k, the effect is significant. You need to keep in mind that it scales exponentially.
The second reason are their skills. Both have skills for attacking cities, wich we'll keep out of the equation, because you should only Rally cities. And that's a topic of its own. They've also both troop expansion by 10%. So you'll only need to compare their other ones. Osman with "Rising Empire" + "Sword of Osman" against Scipio "Military Life" + "Patient Warrior". Skill damage beats troop Buffs. You'll do considerable more damage with one strike than you'd do over time. This makes Osman superior. He's the best epic hero.
For talents the choice is straight forward. Just maximise his blue skill talent tree, ignore the yellow conquering one and distribute the rest to your liking in the red leadership one. First talent you should work towards: "Rejuvenate" - the blue hand. You basically maximise the damage output of his skills and how often you can cast it. You look for Rage and skill damage with your talents.
And who should accompany him? Sun Tzu. That will be another post though. Quick summary: AoE damage, 10% flat damage reduction and skill damage increase.