This is one of the best freewaresynths available. I personally don't like the GUI, but that is richly compensated by the broad amount of features, the good sound and the 64 presets. And after tweaking around you will understand quickly how this synth works.
...but Triangle I is a bit of a different animal... I'm glad I have it and I wish rgc:audio wuz still doin' it to the vsti and mini-host world. Does anyone know if they're developing under a new handle??
No. All this series (Triangle, Square, and even Pentagone) is being totally abandoned for years and years.
Like you I have all the versions of all these wonderful synths, and yes they were a bit different in their results from versions to version... But apart very few exceptions I always use the last version of these old excellent synths created by René G. Ceballos (rgcaudio, rgc was the three initials of his name).