Why was Basel, Switzerland chosen for the First and other later Zionist Congresses to be held in?
The First Zionist Congress was originally planned to be held in Munich (even some of the invitations exist!), which was a cultural hotspot at the time. However, in Munich there was a large Jewish community, some of whom were ambivalent about hosting the conference. Notably many of the more secular of them believed that holding the congress would bring upon them the wrath of the local German population, who had prejudices regarding Jewish foreign allegiance. Alternatively some of the more religious groups saw the congress as a heresy, and Herzl to be a sort of modern false prophet.
So an alternative was searched for, Herzl believed he'd encounter less fears and resistance in Switzerland. Zurich was ruled out because apparently at the time it did not have a good reputation. So Herzl sought out another Swiss city, and Basel was chosen because of its proximity to French and German border, as well as it having a kosher restaurant, which could accommodate the religious representatives.