The 152 is the best option for reason of price/availability/ease to fly.
I've found it very easy to teach the basics in the 152 and the stall spin characteristics require attention without being dangerous. If you have access to a 152 aerobat then you can explore even further (with an instructor on board). A student could be talked through their very first flight without much instructor control input at all. Very defined attitudes and good predictable stability allow it to be trimmed well especially in the circuit. Downsides include limited cabin space and performance issues (which can be of benefit).
I would assume the 172 would be more expensive and is also a little harder to land with just two up front. This would apply equally to a PA28 which i would regard as the easiest aircraft ever to fly.
The tomahawk is a rare aircraft in Australia so only a handful of schools operate them, this could mean a conversion to another type if they sell their only one or you move schools.