Tom is the best expert ...
Assertion of a fact. Where there are agreed criteria then there's no need to hedge an statement.
Djokavic is the current number one male tennis player in the world
When criteria are less certain, or we ourselves are not sure of our facts, or we wish to be modest by appearing to be uncertain of our facts we may use I think
I think Djokavic is the best male tennis played of all time
I think that the population of the UK is 50 million (actually 65 million)
This formulation implies that we are open to correction and discussion
Adding would softens this further, emphasises that we are uncertain
I would think that the population of the UK is greater than 50 million
The I would have thought formulation is normally used in a context where some information has recently been given. Depending upon the context it may imply that we are actually contradicting the information, or that we are expressing surprised acceptance.
I think Tendulakar is the best batsman of all time
I would have thought Bradman had a better record
that was disagreeing, suggesting that by some criteria Bradman is better
The current UK population is 65 million
Oh, I would have thought it was only 50 million
but now I've changed my opinion (this implied but not said)
that was agreeing, I thought it was 50 million, but I accept your statement of 60 million is correct. We could just say
Oh, I thought it was only 50 million
With pretty much the same meaning, the slight difference being that the second case implies it was actively in my mind, whereas the would form could imply that I hadn't really formed a solid opinion until now, but I would probably have guessed 50 million.
As your comment indicates we are indeed into shades of meaning and idioms. In these cases the tone of voice will often differentiate the meaning.
I would have thought Bradman had a better record
Would probably said with a questioning or challenging tone and raised eyebrow.
Best Answer
These two idioms have slightly different meanings:
I don't generally hold you or your skills in high esteem, but on this occasion, I can see you did a good job, and I admire you for it. (Can also be used affectionately or teasingly, where you don't really mean that you think the other person has low skill)
I don't generally agree with you or your arguments, but on this occasion, what you said was correct. Alternatively, I admit that I was wrong and you are right.
As you can see above, one of the expressions relates to what someone has done, while the other relates to an argument or a statement. So, without knowing any of the surrounding context, here's how I would interpret the two statements.
"I have seen you do something that requires intelligence, and I did not previously think you were capable of the task. Good job!"
or
"I have seen you do something that requires intelligence. I'm teasing you that because you are intelligent, I'm implying there's nothing else good about you. I don't really mean it, and I like you a lot. We are good friends."
"I may not agree with you most of the time, but I admit you are intelligent."
or
"I don't think you have many positive traits, however, I admit you do have at least one positive trait: your intelligence." (Alternatively, this could also be an affectionate tease by implying you think the other person has few positive traits when you don't really mean it)
Without knowing the full context of the relationship between the two characters, and the situation where this was said, I can't be certain which of these interpretations is correct, but I'm pretty certain it's one of these meanings.