Answer
The most stable isotope is Krypton-81 and the least stable (hottest) is Krypton-73.
The times for 87.5% to decay are:
81s, 34.5 min., 44.4 h, 6.3 *10^5 yr.
Work Step by Step
The most stable isotope is the one with the longest half life, and the least stable is the one with the shortest half life. 87.5% of each isotope decays in three half-lives, so to find the time, we just triple the half life for each.
