Trials of Osiris - Titan
Both the Striker and Defender have their selling points for the Trials of Osiris, so you should pick the subclass depending on your style of play and your skill level.
The Titan Defender with the Ward of Dawn super is the top pick for the Trials because of the versatility that lets you switch between offensive and defensive tactics.
Forget about the Helm of Saint-14 - no human player is going to be stupid enough to stroll inside the confines of your bubble shield. Instead buff the bejesus out of it. The Glasshouse helmet's Bathed in Light upgrade extends the duration of Blessing of Light and Weapons of Light, although of the two, Blessing of Light is best suited to PvP thanks to the temporary shield it will give you and your fireteam.
The Bastion ability modifier will increase the time Ward of Dawn is active to 45 seconds, so it's worth activating, while the Illuminated modifier increases the benefits of Blessing of Light and Weapons of Light.
Lastly, the Suppressor Grenade will take care of anyone trying to shut down your Ward of Dawn by disabling their abilities - including supers. So it'll also prove handy outside of your bubble.
If you favour the Striker subclass, it's all about the Fist of Havoc. Equipping the Helm of Inmost Light will give you the Recklessness upgrade, that provides the Death from Above perk (aim and activate Fist of Havoc when in the air) and Headstrong (increases range of Fist of Havoc), for free. The obvious bonus is that you can enjoy the benefits of these perks without having to waste a modifier slot on them. So take the opportunity to enable Shockwave to bump up Fist of Havoc's range.