Sets the Standard

I may be biased due to familiarity, but FTB Quests is hands down my favorite quest mod. Easy controls, great UI, lots of customization options, and tons of features (especially considering kubejs integration) make developing with FTBQ a treat. By default, FTBQ is a streamlined questing system with intuitive visuals, but FTBQ can also be a shop, a bounty board, a virtual portal room, an ore processing diagram, a cross-dimensional telecommunication matrix, or even all five at the same time!
As for downsides, all of those examples will necessarily have an FTB Quests look to them. While you can use a resource pack to replace the (admittedly barebones) default textures and add your own images if you so choose, there are still only a few quest shapes and the UI elements will remain the same. There also aren't any per-player "themes" as some other questing mods have - the textures, colors, and quest shapes will always be the same per pack.
To me, the positives greatly outweigh the negatives. (even though it looks like I wrote about the same amount for each... oops) While I'm all for competition, I have yet to see a quest mod that I prefer to FTBQ.
(I might edit this and put some of my favorite implementations; if you want to see them remind me lol)