There's no best, just the most strategic, for PVE u take what mobs dont have high resist against, for PVP u take whatever u want.
If we take only the endgame stage into account and ignore varied class usefulness in different places, Holy is the best element for both PvP (as Seivlas mentioned) and PvE raids as well as some hunting zones, Wind is remarkable for almost any PvE farm, Water is good and rare enough for and in PvE and PvP. Wind for PvE farm: Enchanted Valley, Hellbound (Magic Circle and Ridge), Giant's Cave, Dimensional Warp, Raider Crossroads, Blazing Swamp, Fairy Settlement, Isle of Souls and Nornil's Cave (S2, S3), Antharas Holy for PvE raids and farm: Baylor, Balok, Beleth, Zellaka (party), Atelia, Giant's Cave, Land of Chaos, Pagan Temple, Coral Garden, Seal of Shilen, Fairy Settlement, Dimensional Warp, Dimensional Rift (in Aden) and in PvP versus people who added Dark resistance to 3 armour pieces (almost everyone) Dark only for Octavis raids, Garden of Spirits and The Cemetary Fire for Istina instances, Emerald Square, Fairy Settlement, minor use in some other places Water vs Valakas, Pelline and pve farm in Desert Quarry (Hellbound), Blazing Swamp, Steam Corridor, Dimensional Warp Earth vs Kalios, Lindvior, mobs in Garden of Spirits Dark, Fire, Earth attack attributes are the worst elements to take. (There are resists against them from dyes and epics too.)
