I personally like the apps from Orbx, Contrail and iniBuilds, because they have user friendly UIs, products are easy to update, and they allow me to install my addons on a separate drive and link them with Addons Linker and so keep my community folder tidy.
The worst for me is Sim Market. I have a whole bunch of addons from there, but it’s a pain to keep track of the updates. The only reason I still occasionally buy stuff from there is because they are often the first to release new addons.
The official Marketplace is of course a convenient option but it clutters the community folder. I‘m also not a fan of their user interface and the download speed can be slow.
I think the marketplace is a bit of a missed opportunity. When the sim was released, I really liked the idea of being able to purchase and download all my addons directly in the sim. But due to the reasons mentioned above and the fact that 3rd party addons are (or were) often released much later than on other platforms I started buying from other platforms. Now I‘m back to keeping a spreadsheet to keep track of where I bought my addons, like in the olden days.