During the 1995 World Gliding Champs in NZ we had a number of different towplanes. Wilga, 260hp Pawnee, 235hp Pawnee, Cessna Agwagon, Cessna 180 and 182, Cessna Bird Dog and a Gippsland GA200 Fatman.
The best was the Bird Dog, by far, even though it had the lowest power (210hp?). Nest best was the GA200. Looks like a Pawnee, but performs much better. You can also get dual controls, great for towpilot training. Engine never got above 212F CHT due to great cowling design.
Next was the 260 Pawnee, 235 Pawnee, then the Wilga (I thought it would be better), and then a race for last between the Agwagon and the Cessna 180. I can't remember how well the 182 did, but I do remember helping to lift it out of a large rabbit hole it got stuck in!!
The Bird Dog however was a sight to behold. Good speed for the ASW22's/Nimbus 4's, and fantastic climb rate. With a power on spiral dive it also got down pretty quick. I hear the electric flaps can give trouble, but they really do tow well.
Most of my early towing was behind a 180hp Super Cub. That also towed really well, but Cubs are getting too expensive to buy nowadays and you also have the extra hassle of fabric.
For a new tug, I'd go with the GA200. For a second hand, I'd go with a second hand GA200
or a Bird Dog.
Turbines are great for the no shock cooling thing, but a lot of the components (turbine discs for instance) are lifed on cycles (landings). The more landings you do, the quicker you use up the life of the component. Not good if you do lots of short flights, like say, with glider towing!!
I thought Pik were working on a liquid cooled diesel powered single seat tug but I can't find a link to it at the moment. Now that would be a good way to go.