You resolve in order of most recent reaction taken.
The full text on reactions:
Certain special abilities, spells, and situations allow you to take a special action called a reaction. A reaction is an instant response to a trigger of some kind, which can occur on your turn or on someone else's. The opportunity attack is the most common type of reaction.
When you take a reaction, you can't take another one until the start of your next turn. If the reaction interrupts another creature's turn, that creature can continue its turn right after the reaction.
So we can see here that reactions are instant responses to a trigger. Lets say that the trigger is Bob casting the spell Fireball. Alice, Bob's opponent, doesn't want to take a fireball to the face, and promptly Counterspells, so as not to be burnt to a crisp. Since Bob does want her burnt to a crisp, he decides to Counterspell Alice's Counterspell. So now we have this scenario:
- Bob casts Fireball
- Alice Counterspells
- Bob Countespells
In order for the spell Counterspell to work as intended, the most recently used reaction must resolve first. If we had more wizards available, then we could theoretically have a chain of Counterspells as long as the [number of available casters] +1 (though as a DM, I personally would restrict it to only 4 or 5 reactions triggering off each other)
Since Readied Actions and Reactions use the same rules, we can safely say that a similar chain of events can be perform. Bob Readies an action to attack when Alice attacks. Alice readies an action to attack when Tim attacks. Tim readies an action to attack when Jane attacks. We get to Jane's turn, and she takes a swing at Bob, triggering Tim's Readied Action, which in turn triggers Alice's Readied Action, which then triggers Bob's Readied Action. Then we resolve in revers order, Bob hits, then Alice hits, then Tim hits, then, finally, Jane hits.