Such a question is an open invitation to religious warfare. Everybody loves to participate, but nothing is ever settled.
If the question means “what kind of fencing is the most fun?” then the answer is: it depends what aspects of fencing you enjoy the most.
If you are fascinated by technique, bladework, and tactics, you will probably get a lot of satisfaction from foil fencing. More visceral fencers who want to experience the adrenaline rush of a fast, agressive sword fight will want to try some sabre. Most epee fencers consider themselves practical, no-nonsense sword fighters who rely on as few artificial rules as possible. Enthusiasts of more medieval combat styles, involving armour and heavy weapons, should consider joining the SCA or a kendo dojo.
On the other hand, if the question means “which weapon is the most deadly?” the answer will depend on a lot of factors, not the least of which are the skill of the combatants, the presence of armour, the military and cultural context, and the rules of the fight (i.e.. is this a street fight, a gentlemen’s duel, or open field warfare?). Most swords are highly optimized for performance in a specific environment, and will not perform well outside it. Comparing two swords from completely different historical contexts is therefore extremely difficult, if not downright silly.
Then again, perhaps the question means “which style of fencing is the most realistic?” It must be said that questions of realism have little relevance to an activity that has almost no practical application in the modern world other than sport and fitness. Historically, however, epees have the closest resemblance (among FIE weapons) to real dueling swords, and the rules closely parallel those of actual duels (sometimes being fought to only a single point). Other martial arts with a high realism factor include kenjutsu and some aspects of SCA fighting.