No, not just one. Three sentences blow men away and make them feel miserable and pathetic. If you don't want to ruin them completely, then never say these three sentences.
Men are, actually, quite sensitive in some situations, so even a seemingly harmless remark can completely disturb them and make a short circuit in their brain. 'Metro' magazine identified three situations that will most effectively ruin your day and his confidence.
You don't believe it? OK, try this: when your dear fixes something in the house and puts on a proud face that says "See how I did it masterfully," you give it a glance, shake and say something like 'Mhhm, David / Peter / John (your ex) was a true master in such things'. Actually, don't if you want to live or if you have any plans with him, because you will spend days and days trying to redeem yourself for this cardinal mistake.
Remember – ex-boyfriends don't exist, they were swallowed by another dimension and they are never to be mentioned again. Never. Not even when they're dead.
Another situation is when you recklessly blurt out that Mark / Nick / Alex (his friend) is pretty handsome ... And once again uncomfortable silence will appear, he will look like as if all his hopes are gone, and in the next couple of days you will have to listen to his childhood stories in which the friend in question turned out to be unbelievable 'scum'.
And finally, the least drastic, but still a drastic situation and also unusually painful for any man who has had at least the slightest experience with women - the question ' Do I look fat in this?' Any man with at least slightest experience would wish for any of the previous two situations rather than hearing this question because they know they are absolutely in 'no way out' situation and nothing he can say will remain unpunished... and he can't say anything either.