-Mr. J Shufflebottom-
His weakness is anything addressing his style.
Your hat
Your sense of style
Wear/Wears second-hand clothes
Put/Puts on granny's clothing
A cheap suit
Homeless man's socks
Can't tie a tie
Has/Have worse hair than

-Mrs. Maggie-
She doesn't like being insulted about her age.
Is/Are old
Is/Are deceased
Is/Are stone dead
Is/Are no more
Will soon kick the bucket
Tell/Tells dirty jokes at funerals

-Sir Knight-
He doesn't like hearing about the modern world.
Never watched Star Wars
Still use/uses Windows Vista
Don't/Doesn't own a colour telly
Sucks/Suck at Overwatch
Have/Has only local multiplayer
Can be found in Pokemon GO
Enjoyed Batman v Superman (Credit to Monster the Festive Wolf)
Play/Plays hidden object games (Credit to budder_is_epic)
(You can get an achievement for saying "You still use Windows Vista and never watched Star Wars.")

-Nigel H.-
This guy is really fat, and doesn't like when people point that out.
Your face
Is/Are getting fat
Is/Are dull and ugly
Will always be alone
Has/Have an unsightly face
Looks/Look like a sad lemur (Credit to Caspinator)
Can't exercise because of (Credit to Allosaurus)
A grunting sow (Credit to AlexthePichu)
An English pig dog (Credit to GraniteBeetle64)
Is/Are an ill-nurtured whey face (Can only be used by Sir Knight) (Credit to GraniteBeetle64)

-Grisha-
Grisha is proud of his country, and doesn't like when people dis where he's from or make assumptions about his origin.
Your country
Is/Are not a part of Europe
The Nazis
The communists
Was/were born in (Credit to several users)
Stalin's jockstrap (Credit to GraniteBeetle64)
Is/Are an educated nincompoop (Credit to GraniteBeetle64)
Like a minging peasant! (Credit to The Salt God)
You commoner! (Credit to Fabris)
You ginger! (Can only be used by Albert)
(You can get an achievement for saying "Your country is not a part of Europe.")

-Father-
He may have more health than everyone else, but that's no problem if you can hit his weakness. This one is easy; just bring up anything regarding family.
Your mother
Your father
Your sister
Your son
Your beloved auntie
Your wife
Your husband

-Lo Wang-
As stated in the game, "My only weakness is being weak." This means you need to look for phrases that deal with being weak.
A frightened schoolboy
Was/Were defeated by
Was/Were owned by (Can only be used by Serious Sam)
Was/Were teabagged by (Can only be used by Serious Sam)
Is/Are afraid of my minigun (Can only be used by Serious Sam) (Credit to Kkerns)
Is/Are worthless
Is/Are an ill-nurtured whey face (Can only be used by Sir Knight)
Will be chopped to bits (Can only be used by Lo Wang)
Suck/Sucks at Overwatch (Credit to Aya Shameimaru and LOW WANG)
Defenseless young men (Can only be used by Mrs. Maggie) (Credit to budder_is_epic)
Has/Have no life experience
You pitiful mortal! (Can only be used by Father)

-Serious Sam-
Sam doesn't like it when you mention things that are "too serious." Some of these may not be considered serious by some, but this also refers to things that are complex or difficult to understand.
Have/Has bum cancer
The Meaning of life
The Nazis
Tell/Tells dirty jokes at funerals
Is/Are deceased
The Queen
The Royal Family
Makes/Make fun of the pope
Need/Needs a liver transplant
Is racist (Can only be used by Albert)
And that's racist! (Can only be used by Albert)
The Communists

When you hit Albert's weakness, you see that it deals with "wealth and poverty." Also, his character description asks "How do you make a living?" In short, Albert's weaknesses deal with money and how people make a living.

Your house
A cheap suit
Is/Are worthless
Made a poor deal with (Can only be used by Albert)
Your kettle (Can only be used by Albert)
Wear/Wears second-hand clothes
Is/Are an uneducated nincompoop
Don't/Doesn't own a colour telly
Homeless man's socks
You commoner!
You posh fopdoodle! (Can only be used by Mr. J Shufflebottom)
Like a minging peasant!
You son of a washerwoman!
Cannot buy a stairway to heaven (Can only be used by Father)

This conversation
The Meaning of Life
Has/Have bum cancer
Is/Are an organ in your abdomen
An ordinary pigsty
Will always be alone
Can't tie a tie
Has/Have bad breath
Has/Have no life experience (Can only be used by Mrs. Maggie)
Can't hold liquor (Can only be used by Grisha)
The thing you had for lunch (Can only be used by Nigel H.)
Your pimply arse (Can only be used by Albert)
Is/Are racist (Can only be used by Albert)
And that's racist! (Can only be used by Albert)
Because I don't exist! (Can only be used by Father)
You pitiful mortal! (Can only be used by Father)