I would say outside of CWTN plugins it is kind of hard for me to nail down 1 single plugin that really stands out. The entire MQ/RG package that you get with level 2 has SO MANY amazing features.
I would say honestly the tool that impacts my daily play the most is
Mq2Map. Sometimes I don't want a separate window open to use MySEQ and it really helps to have that resource in game.
But outside of the default features that RG has, I would say if you have the money the CWTN plugins are
amazing. Editing
Kissassist files every few levels was a hassle for me with 6 toons. I like everything to be running as efficiently as possible, and CWTN does this for me behind the scenes and quite honestly plays my toons better than I can. I have 5 of the plugins and would have 6 if the Bard was available.