With the basic meaning "to believe," glauben is the verb most often used for opinions.
Ich glaube, du bist die schönste Frau der Stadt.
Ich glaube, es ist schon zu spät.
By itself, denken can be used to talk about an opinion that was held mistakenly or about which the speaker is not too sure:
Ich dachte, nächste Woche kommt er nach Hause.
Ich denke, wir gehen zu Fuβ.
This verb is used in a sinilar way when the speaker has just made a decision but is not sure about it:
Ich denke, ich fahre morgen nach Hause. (Perhaps he/she is hoping to be talked into staying another day!)