The best is PS/2 because:
-LPC chip on motherboard decodes the packets which mouse sends (as original i8042 keyboard & mouse controller does).
-As for hardware resources only rarely used IRQ12 is required for mouse (mouse uses i/o already used for keyboard: 60-64h).
serial mouse disadvantages:
-mouse sends raw packets to CPU. However transmission is hw accelerated by serial controller, but CPU must decode packets sent by mouse.
-COM port is wasted and its resources.
bus mouse disadvantages:
-CPU is polling ISA mouse card registers by every 30 or 60Hz (frequency depends on vertical refresh of screen so it can be 100Hz or 200Hz too) to check for mouse movement. Inport kind of such card is only probed when there is movement of mouse. This is optional feature of inport mouse which make it similar to PS/2 way of working. The i\o port and irq is required to work.
So it looks PS/2 is the best connector for mouse.