Perhaps the fattest sounding analog modeled vsti's available now are these new ones from rgc, and I consider this a favorite for use with midisax because of the very smooth mono legato it features and the very smooth filter response. If you look around you'll see very few vsti's have a truly smooth mono legato AND respond to pitch bend and you need that for midisax use, or any true mono lead synth.
RGC is not above working with a customer to get rid of bugs either, rene actually personally contacted me on a problem I was having, and that few if any others were, and fixed it literally overnight. That kind of service is literally the extra mile.
...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).