Info status: Published | Jan. 5, 2020, 2:31 p.m.
Power Armor provides between the highest universal PvE protections. For PvP they mostly underperform, compared to PvE effectiveness, if you did not take precautions not to be blown out of the map with VATS. Sneaking can be used surprisingly effectively on power armors to limit your opponent's ability to VATS you, provided that you can nuke their general area with enough violence to deny them time for clear shot.
The strongest nonlegendary power armor in the game would be T51b chest combined with X01 limbs.
The strongest power armor in the game is Strangler Heart Power Armor, which you can obtain by heavily grinding Vault 94 raids. It provides you a poison effect thats acceptable in PvE and ridiculously powerful in PvP - powerful to the point that using it is quite often considered low.
The chest mod of power armor is critical to its overall functioning. It defines the character of given power armor set.
The best chest mod to make grinding experience initially easier is Tesla Coils - it activates electric AoE effect around you which will deal tiny damage to nearby opponents, tagging them for free.
The best overall chest mod is Emergency Protocols - for PvE purposes, it makes you more tanky at 19% HP with Emergency Protocols that you'd be at 100% HP without them.
The most universal mods for legs are Calibrated Shocks, each gives +50 lb carry weight. You might want to swap one of them for Kinetic Servos, that increases your AP regeneration while walking to the levels you get while standing.
The mods are crafted using Stable Flux - a resource that you obtain either from crafting, or as a reward from end-game events. Since the rewards are for participation, you should participate even if you're unable to contribute - you'll contribute by giving the fluxes to the person you want power armor mods from, that's the noticeable cost they have to pay for modding. To craft them, you need to pick up 10 raw flux from plants, and gather a piece of: hardened mass, high-radiation fluids, irradiated meat. Flux plants replaces regular herbs in a nuked area, the three additional ingredients drop from irradiated, nuke enemies.
The lowest level required for power armor is 15, at which you can wield the lowest tier of Raider Power Armor. Getting your own set of low level Power Armor is described here. If you get yourself such a set early, and manage to get someone to mod it for Tesla Coils, you'll drastically boost your ability to gather experience.
The funny trick about raider and ultracite power armor sets is that they have received Jet Packs later than other sets, and the plan to craft the jetpack is kind of bugged, allowing you to place jetpack on any of the limbs.
Since the arms mods are mostly useless, you can add an ultracite or raider arm modded with Jet Pack, and benefit from power armor sets that use two chest mods.