1. I think
This is the most common and general way of giving an opinion. You can use it both informally and formally: I think if you offer a consistently good service to your clients, they will keep coming back to you.
This is a more informal way of giving your opinion: I reckon it will be much faster to get to London by train.
3.In my opinion
4.In my view
These expressions are more formal and are often used when talking about important issues：
In my view, they made a huge mistake in not selling the company when they had the chance.
In my opinion, the Bank of England should not raise interest rates this year.
5.It seems to me
6.All things considered
When you’ve thought about a situation carefully you could use either of these two expressions：
It seems to me that they are spending more money than they need to to attract new talent into the company.
All things considered, I think we made a wise choice in recruiting James.
7. If you ask me
This is used when your opinion is critical. Sometimes, people say this even when their opinion hasn’t been asked for! – “If you ask me,…..” “But I didn’t ask you….!”
If you ask me, she has spoilt her children far too much.
8.To be honest
9.To tell you the truth
10.To be frank：
All three expressions are a way of giving your opinion when you know that people may not like what you have to say:
To tell you the truth, your father was right when he said that you undersold the company.
To be honest, I preferred it when you were blonde.
To be frank, I thought her acting was simply terrible.
11. Frankly speaking
You would use this expression to give your opinion in a more familiar and forthright way.
Frankly speaking, I don’t know what she sees in him.