Check your Collections tab. Not counting those mods earned from the Seasonal Artifact: Weapon mods are mostly purchased from Banshee-44 with some available through Ada 1. The majority of mods for Head, Arm, Chest, Leg and Class Item slots can be obtained randomly from rank up packages with various vendors. Those denoted as “Enhanced” drop from pinnacle game activities such as Iron Banner. Then you have the General mods: • Eight are available through the Garden of Salvation raid but are made specifically for use in that raid alone. • Five can be gained within content from Nightmare Hunts on the Moon. • Riven’s Curse is found naturally on armor drops in the Dreaming City, while it’s sister mod Transcendent Blessing is, if memory serves, obtained from opening the chest in Mara Sov’s throne world after completing the weekly Gateway Between Worlds bounty. • Four mods are available through the Last Wish raid, but can be obtained without actually participating in the raid itself. Two chests can be reached by warping to the Shuro Chi and Morgeth encounters, and each chest has a chance to drop these mods. • Four mods are available through the Scourge of the Past raid. • Eleven mods listed as dropping in the Menagerie can be obtained there. The four mods that drop from the Crown of Sorrows raid can be earned in that raid, but also have a rare chance of dropping from Menagerie completions. Either way, be sure to bring an upgraded Chalice. • Dawn armor mods are now available through purchase from Banshee-44 on a rotating basis (occasionally) as they are no longer obtainable from their original source. • And certain Worthy mods can be obtained from upgrading Rasputin’s bunkers, while others are rewards for completing Seraph Tower events. Aside from all those groups are mods that were available to be used on older armor pieces from before the implementation of the current ‘Armor 2.0’ system. These mods have been deprecated and can no longer be obtained, as the armor they were compatible with no longer drops. The only way to equip and use such mods is to equip old armor you had prior to the switch. (΄◉◞౪◟◉｀)

Depends on your play style. What weapons do you like?