NOTE: this does not refer to the case of readying a dispel magic spell 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 the dispel magic spell. A readied spell is cast as normal on your turn, but its energy is held (requiring your concentration) until the trigger occurs, after which it is released with your reaction. Would this classify it as an ongoing magical effect (explicitly called out in the description of the dispel magic spell)?
The rules on spell duration (PHB p. 203) explicitly split spells into "Instantaneous" and "Concentration" sections, mentioning under the latter that (emphasis mine):
Some spells require you to maintain concentration in order to keep their magic active.
However, the official Sage Advice rules answers column for March 2016 says, under the question "Can you use dispel magic on the creations of a spell like animate dead or affect those creations with antimagic field?" (bold for emphasis mine):
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).
How do these rules interact? Can dispel magic be used to stop a readied spell from taking effect before the trigger occurs?