Bennett is a Pyro support-type character. The scaling on his skills are on the lower side when compared to the other characters so he may seem weak at first. The trick is to never hold his Skill and always tap it; this way you gain energy immediately and proc reactions more often. His passives reduce his Skill cooldown so much that you can effectively tap it every attack string. He also provide a huge attack boost to your party, and his passive to lower the time of expeditions can come in handy.
C1 is extremely helpful if you can get it to make the ATK boost from his Burst more consistent and powerful.
Depending on your playstyle, you can either build him by focusing on his physical damage for solo playstyle, or focus on Pyro for reaction and teamwork play. It's recommended to go with Pyro damage since it deals a lot more damage if you use him with any Cryo or Hydro character, and the cooldown of his elemental skill is pretty low. Use 4 piece Crimson Witch of Flames set and get Pyro Bonus% on the cup, and crit rate crown piece if you don't use a weapon with crit rate, otherwise, it's crit damage mainstat on the crown piece. For physical damage version, use 4 piece Gladiators Finale with physical bonus& on the cup.
The best weapon to use for him is The Black Sword, since it's a really good sword in general because of the passive as well as crit rate secondary. If your only choice are the craftable ones, Prototype Rancour is the better choice. Any of the 5 star sword is also a good option, Aquila Favonia being the best out of the 5 stars.
If you only want him as a support for his elemental burst, then you don't really need to invest a lot into getting good artifacts main/sub stats, since his burst only uses his base attack value. This means that you can bring him into your party with a level 1 artifact set for the set bonus, and he will still be useful. Best support artifact set in general is Noblesse Oblige because it buffs your party. If you find yourself often feeling like you need his burst to be up more often for the attack boost or heal, then consider using 2 piece The Exile with 2 piece Scholar instead. Be sure to get energy recharge on the hourglass, and healing bonus on the crown piece.
As for weapons, any weapon that gives energy recharge will be good.
ATK is increased by 20%. Triggers on taking DMG: the soul of the Falcon of the West awakens, holding the banner of the resistance aloft, regenerating HP equal to 100% of ATK and dealing 200% of ATK as DMG to surrounding enemies. This effect can only occur once every 15s.
As a support, you simply can't go wrong with bringing bennett to any party, especially if you have another Pyro on the team for elemental resonance. His attack buff from Burst will always remain useful, inflicts you with Pyro status so you can cleanse debuffs, and provides a strong heal.
As a main DPS, you'll want to bring characters that you can combo off of, like Kaeya/Chongyun/Diona for Cryo so you can consistently cause "Melt" reaction, or with Xingqiu for constant "Vaporize". Try not to use characters that wants to be on the field for long like Razor, because you will end up fighting for the main dps spot.