NOTE: this does not refer to the case of readying a Dispel magic to stop a spell from being cast as in this question, but rather using Dispel magic to stop a readied spell from being cast.
While writing this answer, I ran into an interesting corner case regarding Dispel magic. Given that a readied spell is cast as normal on your turn, but its energy is held (requiring your concentration) until the trigger occurs, after which is released with your reaction, would this classify it as an ongoing magical effect (explicitly called out in the Dispel magic description)?
The rules on spell duration (PHB p. 203) explicitly split spells into instantaneous and concentration, mentioning that (empasis mine):
Concentration (PHB p. 203)
Some spells require you to maintain concentration in order to keep their magic active.
Whenever you wonder whether a spell’s effects can be dispelled or suspended, you need to answer one question: is the spell’s duration instantaneous? If the answer is yes, there is nothing to dispel or suspend. Here’s why: the effects of an instantaneous spell are brought into being by magic, but the effects aren’t sustained by magic (see PH, 203).