These are some of the various online tools I often use when developing my datapacks.

By far my favorite tool! By far the best command generator out there. It has (almost) everything!
Minecraft Tools
Has a few command generators, but I use it mostly for the tellraw and book generators. It has a very intuitive interface. Plus, you can even save your commands!
Has very nice recipe and advancement generators. They are both intuitive and fairly easy to use. I use this site almost exclusivly for the recipe generator.
The ultimate chad when it comes to miscellaneous generators. They have loot table, predicate, and even worldgen generators. Very, very helpful!