C#: most readable string concatenation. best practice

Possible Duplicate:
How should I concatenate strings?

There are several ways to concat strings in everyday tasks when performance is not important.

• result = a + ":" + b
• result = string.Concat(a, ":", c)
• result = string.Format("{0}:{1}", a, b);
• StringBuilder approach
• ... ?

what do you prefer and why if efficiency doesn't matter but you want to keep the code most readable for your taste?