"I would say" - Would be used before you provide your opinion on a subject. "I have to say" - Would be used if you were going to compliment some one or if you were to comment on something that surprised you. For example "I have to say, I didn't think you could do it, but you did!" "I should say" would normally be used to place emphasis on your agreement with something. For example. Speaker A: "Do you think he is right to comment on that?" Speaker B: "I should say so, he is right about everything"