Two arguments can be made, one for a monarch and another for an elected executive. In either instance, we end with a sole ruler or source of executive power over the people. Montesquieu demands that the highest power be held by a single individual so to distinguish between the executive and legislative and to prevent the legislative or any part of it to assume any form of executive power.
In the sense of a monarch ruling a government, according to Montesquieu, “…this branch of government…is best administered by one than by many.” In this instance, a monarch comes to power from either birthright or conquest, in which case there is one sole undisputed ruler who assumes some executive ruling power over his/her society. The main weakness to this argument is that a sole ruler is highly susceptible to corruption and inclinations of abusing its power, the very thing Montesquieu aims to avoid in keeping the source of power down to a single person. However, the safeguard is that the threat will only come from a single person who can be suppressed, if need be, by the greater numbers of the legislative.
The argument for the elected executive is as such: that a highly capable (at least on the surface..) individual who is chosen by peers can be trusted to provide rule over a society. The qualifications will be somewhat present in the candidate in that it has been chosen by the people who it will rule over. It should be understood that the people will not, suicidally, chose someone who will only suppress them constantly and treat them as if they were in a prison camp. The power will still be vested in a sole person, only that this person will be chosen and trusted by the masses instead of being one who was fortunate enough to either be of a royal bloodline or ambitious enough to overtake a ruler and claim the crown for his/herself. One difficulty in this scenario would be the duration of rule for the individual. It’s a trivial issue but to enough people, it still is an issue. Also, there is something in which Montesquieu directly fears, that being that while the candidate is being chosen, the role of the executive and the legislative are vested within one massive population. While deciding on a ruler, chaos could run amok over something simple and there would be no way to check the power. Unfortunately, I cannot provide an answer to that problem, but I hope someone can..