Why using AES in ECB (Electronic Code Book) mode a bad idea? Why using AES in ECB (Electronic Code Book) mode a bad idea? is CTR mode better? Given only these two can be used for parallelization.
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.