StrongPlant,
Kudos on the 'Delicious mystery' tomato! They look amazing.
As regards disease resistance, it would be hard for me to say off the cuff. There are so many diseases, most are susceptible to one or another, and to really say something is disease resistant, you need to grow it many years. I could look through my files and tell you which OP's I grew only once because they were very disease prone. But this might also be misleading in different conditions. You'll find people here who have far more experience than I do.
Several years ago I made a cross between the two most disease resistant plants that year - Rozoviy Flamingo heart and Al Kuffa dwarf. Last year I grew out the F2 and in that cold year and crowded conditions they did no better than the rest.
What is the disease that troubles you the most in Serbia?
We get grey mold (Botrytis) here, especially when it's cold and wet. A little Early Blight (Alternaria) for some on the lower leaves but not usually through the canopy. Either of those on the stems can be fatal, and usually is by end of season. And when we have heat and humidity midseason, the mildews are a problem - no resistance to those in anything I've grown so far.
On the scale that I'm growing, leaf disease (except for the mildews!) can be controlled by routine sanitation - remove the diseased leaves. For stems affected, dipping the pruners in bleach has worked well.