Most reasonable of phylogenetic trees as consistent with available along with anticipated real-world complexities.
The simplest of trees is that which requires the fewest number of steps to generate. This would be the fewest number of mutations or changes but not necessarily those that are most likely to occur.
A maximum likelihood tree, by contrast, takes into account such issues as likelihood of occurrence of specific mutations to generate a more realistic if not necessarily simplest of trees.
In other words, KISS is not always the (correct) answer!
Compare with maximum parsimony.