I would say in terms of "essential" plugins, they’re mostly all built-in. Logic covers a lot of ground and does so very well.
For mixing plugins, you have to know what gap you’re filling (workflow, more accurate analog emulation, etc.) or what specific vibe you’re going for. Quality mixing plugins tend to cost a lot of money so it quickly gets into diminishing returns. I like FabFilter, IK Multimedia, Kush Audio, and Softube.
Instruments and sounds are where logic can come in a little weaker in my opinion: depending on the type of music you want to make, Arturia, u-he, XLN Audio and, as mentioned above, Spitfire Audio make great virtual instruments / soft-synths / sample-based instruments.
But yeah – Logic mostly has top-notch stuff. If I could do it all over again I’d spend most of the money I spent on plugins on instruments, monitoring and workflow.