I'd second the Robin 300/400 series as being close to the ideal. You have to factor in the fuel burn as well as the 'ooomph' and in that I think they are very efficient. The DR400 with a VP prop is an excellent performer in this respect and the undercarriage is far more sturdy than it looks.
If you can ignore the initial investment, the best tug I've ever experienced was the Turbo Bravo at Schaenis in Switzerland. It's a mil-spec two-seater with an 'experimental' 360shp Allison turbine in the front instead of the normal 4-cylinder Lycoming... The first launch was in an ASH-25 and the acceleration was so rapid we got airborne in negative flap! I've had 1,500fpm+ behind it in a single seater; the best thing is that it was actually cheaper to launch to height as a) it ran on turbine fuel and b) the cost/min. was high but you needed very few min. on tow. It could also fine off the prop, pull the dive brakes and be ready to go again on the ground from a 7,000' launch in about a minute. Class.