I am using Canon 550D and is going to visit Greece. What lens should I bring if I can afford just one? Appreciate your comments.
It depends on what you are doing, but for general purpose shooting, your best bet is probably to take your budget for lenses and subdivide it to get two cheaper lenses that will cover whatever range you will need for what you will be shooting unless everything you will be shooting is the same size and distance from you (or proportional.)
In general, the quality of one single lens covering a large range of focal lengths will be inferior to two lenses that total up to the same cost and cover half of the range a piece. This is due to the optical compromises that have to be made for a lens with a really long range of focal lengths. Not that you can't get a lens with a long set of focal lengths that is reasonable quality, but it generally costs far more. For example, Canon's 100-400mm f/4-5.6L is a very nice lens over a long range of focal lengths, but it is also $1600.
To get any more detailed than this with an answer, we'll really need more information about both your budget and what you plan on shooting while you are there, as well as if general usefulness after the trip is a consideration.