Which of the following is the weakest hashing algorithm?

I think this question - which of the following is the weakest hashing algorithm is not really meaningful because there isn't a linear scale where algorithms can be rated.

Non-cryptographic hashing algorithms are weak in the sense that they don't have the expected properties of cryptographic hashing algorithms. So if you're looking for hashes that are completely broken as cryptographic algorithms, look for non-cryptographic hashes, such as CRC. The simplest hashing algorithm is parity, which with a single bit of output can't do miracles.

But algorithms that are designed as cryptographic algorithms are usually not broken in the sense that all the expected properties are violated. Rather, there are specific ways in which some expected properties are violated. Typically, algorithms that were once considered good cryptographic algorithms remain strong against preimage attacks but no longer have collision resistance. This is the case for MD4, MD5 and SHA-1.