Interestingly enough, I didn't forget any RE I bought, I simply uninstall them for some time if I don't use them much. Mainly for less clutter while browsing for devices. Then there is stuff I try to forget on purpose. I own just about 30 REs though, so there is less room for forgetting something.
The ones I truly forget about most of the time are both filer REs I own. DFilter and AF-4. I bought them since I use a lot of sampler patches from various ReFills, but I wanted another option to filter them, but instead I still use the NN19/XT filters, the ones in the mixer or Pulverizer. Weird me.
Overall, most of my RE purchases where based on using only Reason Essentials for about 2 years, before upgrading to Reason 8 again 3 months ago. The Echo went back to be my main delay. Pulverizer for many applications. The full featured mixer also kinda made some REs obsolete in theory, but I still use them quite a lot, e.g. EVE bundle, but there are many REs I own that I bought to compensate missing features or devices in Essentials.
555 posts... that deserves a beer I guess...