As a teacher, I would have to say that one of my favorite things to get from students are pictures. I love it when students have drawn their own pictures to show what they really enjoy that I have taught them. I also got some letters earlier in the year from some of my 4th grade students as part of one of their class projects. The project was called "I Appreciate You" and each student in the class had to choose one teacher that they have had over the years and write a letter about what they have done to help you as a student. I got two letters about a month ago and I think that was my best day of being a teacher
The more personal it is, the more it will mean to your teacher. I guess that's what it comes down to, in my opinion.