plug'n script is the best plugin ever for learning how to code plugins. you won't get very far performance-wise, but you isntantly get started without any knowledge about coding and learn all the important basics and the presets are just great ressources for learning how to code in general.
i don't know how long i use this synth already... it's still my go-to synth when i want a fast and inspiring workflow that gets me into trance as well. whenever i install my daw on a new machine synth1 is one of the first 3rd party plugins i install.
for a free effect this is a must-have. it corrects pitch even better than the "real" autotune, the only downside is that it theirfore distorts the sample quite a lot, but if you go for a robotic voice anyway it doesn't matter, does it? i especially love how easy it is to get to new, wide and unique voices with the formant knobs, transposing and panning. singing with this plugin feels like a bob ross painting. the ultra hard pitchshifting is also useful if you wanna make your voice sound like an eguitar. just use this plugin on its maxed out settings and put any huge guitar amp VST behind it and you feel like a skillful guitar solo.