during which phase of growth are bacteria most affected by antibiotics?

I would say the log phase because that is when they are generating enzymes and cellular structures.

Most antibiotics act by inhibiting formation of bacterial cell walls, protein production, DNA replication, and cellular metabolism. So it is best to inhibit these things when the bacteria need them most, when they are first beginning to grow.

It's a toss up between log and lag. In the perspective of the lag phase, this is where the bacterial cell is adjusting to its environment and producing enzymes and carrying out metabolic processes that would be favorable. It is desirable that antibiotics strike here because they wont even get a chance to reproduce is it cannot adapt to the environment.