It really depends on the agency. As for ease of travel, Slovakia, Hungary, Austria would be the simplest since they are close. Romania, Croatia and Bulgaria are quite a bit further. Road conditions in all those countries are good, Bulgaria being the least developed.
I guess it depends on your time and driving stamina. Why not take the trains though? They are extensive, frequent, and will leave you far more rested than driving. Or even a combination. Drive in the Czech countryside for a few days visiting small towns, then drop the car off in Bratislava, taking the trains from there. Tons of possibilities!