Which is the simplest cryptographic algorithm which is close to commercial-level security?
I would say MiMC is the simplest block cipher with plausible security. The idea is to cube the state, add a random constant, and repeat. This is typically done in a large prime field, but it is trivial to implement field arithmetic in any language with big integer support. Here's a Python implementation:
def mimc(x, p, k, constants):
    x = (x + k) % p
    for c in constants:
        x = (x**3 + k + c) % p
    return x

Notes:

x is the input.
p is the (prime) order of the field.
k is the block cipher key.
constants is a list of r random field elements, where r is the number of rounds. (The paper uses r - 1 constants, since adding one in the final step does nothing for security, but it doesn't hurt either.)
The authors recommend r = ceil(log(p)/log(3)).
The field must be one in which cubing is a permutation, which holds if gcd(3, p - 1) = 1.