1. I think
This is the most common and general way of giving an opinion. You can use it both informally and formally.
例句：When you're older I think you'd be better equipped mentally to cope with whatever happens.
2. I reckon
This is a more informal way of giving your opinion.
例句：I reckon it was the best office party we have ever had.
3. In my opinion / In my view
These expressions are more formal and are often used when talking about important issues.
例句：In my view, our performances at the moment have been affected by tiredness.
例句：In my opinion, the Bank of England should not raise interest rates this year.
4. It seems to me / All things considered
When you've thought about a situation carefully you could use either of these two expressions.
例句：It seems to me that this crime was foreseeable and that this death was preventable.
例句：All things considered, I think we made a wise choice in recruiting James.
5. 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!
例句：If you ask me, she has spoilt her children far too much.
6. To be honest / To tell you the truth / 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.
例句：To tell you the truth, your plan is unrealistic.
例句：To be honest, the house is not quite our style.
例句：To be frank, I don't like him.
This is used to emphasize that you are giving your own opinion.
例句：Personally, I couldn't care less whether the Giants come or not.
8. As far as I'm concerned
When you realize that other people may not agree with you, you would use this expression.
例句：As far as I'm concerned, that is the finest painting Picasso ever painted.