I guess, "must-have" depends on what you want to do,    Here are some that I use in all kinds of games (that aren't D&D or PF),     
MotD (message of the day), 
    TokenMod if you do GM-prep involving setting up tokens / bars / auras / status markers, 
    ColorEmote to let your PC type in chat with their Character Picture large and a colorful background, 
    Recursive Table to make more powerful rollable tables, 
    Marching Order / Carry Tokens / Bump / Random Rotate.... for different types of token movements, 
    Roll20AM / SimpleSound / or keith's new audio one.... for controlling the Jukebox with macros/chat triggers, 
    ChatSetAttr if you want to automate/macro/chatbuttons for changing Character stats (can work with any char sheet, any game system that has Attributes in roll20) 
    Director... New for theater of the mind,     <a href="https://app.roll20.net/forum/post/12437726/script-director-a-roll20-mod-to-handle-theater-of-the-mind-play" rel="nofollow">https://app.roll20.net/forum/post/12437726/script-director-a-roll20-mod-to-handle-theater-of-the-mind-play</a>     
    And some more obscure (not in the One-Click) code snippits I favor, 
    Waypoint Patrol to have tokens marching in a pattern/loop around the map such as a flying dragon or guard patrols, 
    h2-chatfont for the option of large emphasis text or oversized Emoji in the chat, 
    AnnounceNats to display a special colorful message when someone rolls 1 or 20.    Hope this helps! If you don't have any API mod and this is your first one, get MoTD Message of the Day and start there.