There is no simple answer to this as many players rotate each season, some of whom are critical in achieving endgame tasks. Most are activity-specific, such as the advanced relay defense mods, which are great for boss destruction in the Garden of Salvation but not so much elsewhere. It all boils down to the class in which you're playing, the guns you're wielding, and the construct you've chosen. To put it another way, it's best to collect as many mods as possible rather than focusing on a single one. Also, whatever you pick, ensure that it enhances your playing style and tools.