Yea, Mt Framework 1.x was very smooth engine,BUT its freakin messy as hell, For example arc files are numbers, same as other files, well, theyre inspired by some japan console engines whinch used aswell this technique, idek how they could navigate in this mess. Anyway this engine really pisses off modders becouse you can only extract archives, import some models, convert textures but thats all, you cant get animations or even sounds, only swap them.
But my favorite engine quality>size is Avalanche. Thats becouse models are very well optimized (for comparsion usual model have around 400% more size than those), also they have different rendering technique than others, they uses 2 UV channels + vertex color. That means over 100 models uses only 2 same textures about size 80kB so they does not use big textures like other engines with over 7GB and more. Thats why Just Cause 2 have only 3.6GB, but extremly large and immersive enviroment. Avalanche devs have my respect.