Overall, I like to use melee combat only against enemies that are easily defeated without a lot of Vitality loss, or if they are weak and can be easily finished off. Otherwise, signs are a better way to inflict damage as well as to do it from a safe distance. Prolonged exposure to melee combat only invites being hit by a dangerous status effect.
In the early game, you will need to use Strong or Fast styles against the correct opponent, or you are unlikely to hit at all. Group styles hit all types of opponents provided you have 3+ opponents. Sometimes, but not always, two opponents will be enough.
When Dexterity improves, you may have enough of an Attack rating to use the "wrong" style on a target. That is, you have a better chance of using Strong style against an opponent that normally requires a Fast style. Even Group style can then be used against a single opponent.
You can use Steel against monsters and Silver against non-monsters/humans, but the damage is much lower and the chances of a critical hit special effect such as Bleeding or Pain is either zero or very low. You still score hit reactions, however.
Therefore, IF you are relying on other means (such as Igni) to kill or severely injure enemies, then theoretically you could develop just Strong Steel or Strong Silver as your primary attack. You would have to wait until the late game to start using it against Fast style opponents. In the meantime, you would rely on level 1 styles on your targets.
There is little point in doing this at levels 1 and 2 because you will have enough Bronze Talents to buy all the non-useless Bronze skills anyway. You won't have enough Silver Talents to buy all level 3-4 skills, so focussing on just one or two attack styles at level 3 will free your Silver Talents for investment in the generally more versatile and useful Signs.
I recommend Strong Steel and/or Strong Silver because these cause Bleeding, and bleeding works on a percentage of current enemy Vitality -- The more they have, the more they lose to bleeding. Also, Strong styles do more base damage than Fast styles.
Another reason to limit your investment in weapon styles is because they cost a lot of talents for redundant effects. Each of the specializations at level 1-3 (e.g., Deep Cut for Strong Silver) apply only to the particular sequence of blows that the level adds. For example, Deep Cut II only applies to the attack sequence added by Strong Silver level 2. In contrast, investments in Incineration for Igni or Poison for Yrden applies to all types of enemies (as long as they are not outright immune).
Yet another reason to limit investment in high-level weapon styles is the flow of combat. Especially when you are fighting multiple opponents, you will often have to reposition, which breaks your attack chain. If the enemy parries or dodges, or if you are hit, your attack chain is also broken, making it harder to reach the level 2, 3, or 5 sequences unless you are fighting a single opponent that you can keep neutralized in hit reactions.
Cut to the Jugular, Deep Cut
The basic Strong attack already includes a chance to inflict Bleeding, but you will still want to maximize your chances of a bleeding wound because bleeding damage is calculated based on a percentage of current Vitality. As the game progresses, you will do more and more damage with bleeding because enemies have more Vitality.
Also, bleeding lets you limit your engagement against dangerous opponents such as Bloedzuigers as you can withdraw while they bleed, and re-engage only if they are not dead. If they are weak after bleeding, you can use a sign such as Igni to finish them off from a safe distance.
In the late game, it is definitely worthwhile trying a Strong style on any opponent to get them bleeding.
Paralysis / Crippling Pain / Sever Sinews
The basic Fast attack already includes a chance to inflict Pain. There isn't any priority in further increasing the chances of inflicting Pain because the Pain status is cancelled when the target is hit.
If you are chaining your attacks together, Pain means you probably can't be interrupted by the target. But since you generally cause a hit reaction in the opponent anyway, this is somewhat redundant. It can, however, help you ensure long uninterrupted chains on a single target. The damage output might still be slower than using a Strong style, however.
Where Pain really shines is when you are one-on-one and there are no other hostiles. When the target is in Pain, during the seconds that they stand idle, you can do something else like drink a potion.
To save some Silver Talents, you might want to skip the level 3 upgrade if you develop a Fast style to level 3. Since you have enough Bronze Talents for everything, you might as well pick up these skills at levels 1 and 2 at some point.
Sever Sinews increases the chances of scoring Pain on the opponent when they are very weak (25% Vitality). Obviously not important to get unless you have the corresponding Paralysis skill.
Critical Hit / Precise Hit
Maximize this. Precise Hits inflict massive damage based on a percentage of enemy Vitality. This is probably what happens when suddenly all your opponents drop dead even though damage with Group Styles is weak. It is still not as reliable as Strong styles for damage output, however.
Crushing Blow / Hail of Blows / Half-Spin, Mortal Blow / Flash Cuts / Tempest
These are the basic damage-increasing enhancements to the Strong / Fast / Group styles, and generally do not have a high priority compared to scoring a critical hit status effect like Bleeding or Knockdown. You will want to get them eventually, however, as a matter of course.
Massive damage increase when your Vitality is extremely low. I prefer to to use tactics that prevent losing that much Vitality, but theoretically if you are in a not-so-dangerous place and are willing to leave your Vitality low, this helps a lot. However, if you are in a not-so-dangerous place, then Bloody Rage probably wouldn't be necessary either. Skip this skill.
Trip / Knockdown
Critical to get very early for at least level 1 Group Steel and level 1 Group Silver. When you are fighting multiple opponents, achieving Knockdown helps you neutralize their threat and focus on any critically dangerous opponents first. Also, the Knockdown is usually long enough that you can use a Finishing Move on at least one opponent. It is pretty much pointless to upgrade Group styles to a higher level without immediately also getting Trip / Knockdown for it.
Massive damage increase for Strong Silver IF the enemy is burning from Incineration. Incineration plus Patinado plus ongoing Bleeding from Cut to the Jugular all adds up to Agni and Strong Silver being important skills to improve. Since monsters tend to be more dangerous than humans, Strong Silver is probably the best choice of all the fighting styles to develop past level 2.
Massive damage increase for Fast Silver IF the enemy is Blinded. There aren't many ways to achieve Blinding reliably or frequently, so you can probably skip this skill.
Level 4 Power-Up Attacks
All the level 4 skills give a power-up attack that does massive damage. However, actually using these also costs Endurance, and there are much more useful things you can do with your Endurance. Just setting up the attack is tricky without Quen to protect you while you are doing it. I recommend saving yourself the headache and not developing any of these except as a stepping stone to the level 5 attack.
Level 4 Strong Steel is the only attack with swords that has the Dislodge Shield ability. This is normally available only with light axes. However, it is not generally a critical effect to have since having a shield doesn't help enemies against Igni, Aard, or Yrden. It does protect them from being constantly pushed back by combat, however.
Level 5 Attacks
All the level 5 skills are a big attack chain for massive damage, IF you can complete the long chain to get there. We recommend spending your Gold Talents on other things such as percentile increases to Vitality and Endurance.