Teacher recommendations can make a difference. Frankly, however, the vast majority sound quite a bit the same. After all, few students are going to request references from teachers for whom they did lousy work. Most teacher “recs" mention what a fine (or reliable, conscientious, hardworking, intelligent, etc.) person you are. While these very typical recommendations don't exactly hurt you, they don't help a whole lot either, and—at the most competitive colleges—that can actually do some damage, because you won't stand out in a tough crowd.
The best recommendations are those that stress how UNUSUAL and OUTSTANDING you are (e.g., “she has perhaps the keenest grasp of mathematics I've ever encountered in a high school student," or “my youngest-ever poetry prize winner ..."; “... she did more community service than any student I've ever worked with ....," etc.). The best ones also include brief examples or anecdotes to back up their claims.