I’ve played all 9 professions and kept swapping between them until I hit engineer and now exclusively play engi in open world/meta events, pvp, wvw, and fractals/raids.
Wvw: mentioned above but holo for roaming (or flamethrower scrapper for memes and plenty others), flamethrower/bomb scrapper for Zerg dps or support scrapper for Zerg.
Open world/meta: holo or scrapper. Holo more engaging, scrapper for laziness.
Pvp: plenty of builds here, again holo or scrapper mainly.
Fractals/raids: power holo for most but also decent condi holo (although a lot more effort than say a CFB). I play cholo for CM100 then power for rest. I also have legendary armor so very easy to swap stats/runes.
All in all, guardian has the most meta and flexible builds but since you mentioned engi, figured I’d comment on that since I main it. Most builds can be picked from metabattle, snow crows or discretize and tweaked as you want.
Good luck!