I'm still on Logic Express myself and quite frankly have absolutely no reason or desire to upgrade.
The stock Logic comps are not bad in the least.
Over the years I've found a bunch of recipes for Logic compressor presets that I will put up against any comp plugs that I own.
Someone mentioned Klanghelm's MJUC earlier; hands down one of my favorite character comps.
I own everything that Tony/Klanghelm has released and his free plugins (MJUC Jr, DC1A, and IVGI) are a must have.
I use DC1A on every session in fact.
His VU meter plugin (VUMT, $9 +/-) is absolutely essential for me personally; it makes gain staging in Logic SO much easier.
For reverbs I agree 100% on the Valhalla plugs; they are shear awesomeness and I doubt you will ever outgrow them.
I use Plate, Room, and their freebie, Freq Echo.
A good free reverb that I still use on occasion is "Ambience" by Magnus (http://magnus.smartelectronix.com/
The presets were developed through the Smartelectronix community and some of them are REALLY good.
Another freebie is TAL-Reverb-2 (https://tal-software.com/products/tal-reverb
The plate reverb algorithm is good enough that I still use every once in awhile.
TAL has a bunch of other free reverbs on his page but this one really connected with me.
As far as other free plugins go there are absolutely hundreds of them out there and some of them are really good.
I worked for about 5 years with nothing but the stock Logic plugins and a bunch of freebies.
"Progress isn't made by early risers. It's made by lazy men trying to find easier ways to do something." ~ Robert A. Heinlein