Why using AES in ECB (Electronic Code Book) mode a bad idea?
ECB is bad because identical plaintext blocks result in identical ciphertext blocks. Encrypted data is therefore not pseudo-random.

This is how Tux looks like after ECB encryption:



CTR mode doesn't have this problem. Data encrypted with CTR mode is pseudo-random and doesn't show any pattern. So CTR is much better.

CBC and CFB can also decrypt parallel, but not encrypt.