You can do what you like! Some people level cap by playing all the overworld stories, daily dungeons (Group Finder bonus or Undaunted pledges), PvP (slow), or grinding spots. So, it's Skyreach - if you're using the Mythic Aetherial Ambrosia with the double XP event bonus (plus full training gear, ESO Plus, and the 1 group member bonus). Ring of Mara would give even more. You need a competent AoE DPSer and self-heals/shields to not die. You should make a full circuit around the main path of the dungeon, aggro'ing mobs and avoiding minibosses (namely the purple ghost). You stop in three places as you make your way around (never in the starter room, but North, then East plus!!! the chest, and finally Southwest before the starter room). Be smart in positioning yourself behind columns or around corners to make all the adds come to you to stack and burn them down. Also be sure to dodge roll out of multiple red AoEs if they pop on you, else you'll likely die. You can do this reliably every 6 minutes. Run out after each sweep and have the group leader retoggle the instance (vet to normal then back to vet). You get over 2mil experience in a 50 minute run. The DLC can be done in any order, as you like. Some of them will make more sense if you have completed the main quest at the Harborage with the Prophet.

I would recommend veteran dungeons and non-DLC version I dungeons at CP160, normal trials with a good build at CP200, and veteran Malestrom Arena and trials at CP300.

You can join up to five guilds and leave at any time, so I's recommend doing that to some and learn how to judge them for yourself best on how helpful you find them.