Pretty much all the name brand tractors out today are good machines, the JD790 is an older design, well proven and very popular. The
B7800 is a lightweight ladder frame design tractor that is also proven and popular. But both have differnent characteristics. The
B7800, being a lightweight tractor is suited for mowing, in fact it is an excellent mowing tractor; it is also a very small tractor for its horsepower so it will fit into places a larger machine won't fit into. On the other hand, if you want to pull a large box blade with the scarifiers down, then the
B7800 will not have as much traction as the heavier JD790. I believe the JD790 is also a bit larger than the
B7800. The Kioti tractors, as a general rule, are built as heavy tractors, they are well refined, especially the CK and DK series tractor. Which Kioti are you considering? I'd suggest the CK30. It has a very good front end loader and good ergonomics. If your tasks are suited to a large and heavy machine, the CK30 would be a fine choice.
As to what is the best made, honestly all popular brands have lemons and all popular brands build good machines. For a 30hp tractor, I would trust any of the following tractor manufacturers: Case-New Holland, John Deere, Kioti, Kubota, Mahindra and Massey Ferguson.