for one

As the first in a number of examples or reasons.

“I don't like this carpet - for one, it doesn't suit our lounge room at all and it's also much too expensive.”

As a single given example, often the most obvious or important one and almost certainly enough to sway the argument, of several reasons for something.

“I am ashamed of you, son. We do not condone theft or violence, Ryan. It is, for one, illegal, and may as such land you in jail or lead to other punishments.”

As one person among other people who may not hold the same opinion.

“I know you may not agree but I, for one, think we should get rid of this unfair piece of legislation.”