Ross has given the physical limits (+1), which can maximize rawbits per track. Apart from this, there are some considerations.
If you use DMA, the ideal limit is 46950 bytes/s, but you can poll DSKBYTR with the CPU to go higher. (If you do this, disregard the rest (on 68000 only) as you will waste 100% CPU on plain loading.)
On top of this, you can use better encoding than MFM and GCR to almost reach 1.5:1 encoding instead of 2:1. (You can never reach 1.5:1.)
And the CPU can easily decode such a low datarate, so on top of this you can add a compression algorithm. On 68000, it must be very simple to make a gain, though. Because if you fill up the CPU cycles, like in Soviet Russia, the hardware will be waiting for you
On accelerated Amigas, you can do all of the above. If accelerated enough, it will be close to the physical specs of the medium+highest compression ratio. (Edit: A very good test case for stock A1200, at least I think there have been no attempts in this direction.)