That's a tough question. The basic design has been consistent throughout all the different models. Version 1 is a classic, a nice simple design that could translate into the real world (not the most practical design mind you, but it could be a real vehicle), Version 2 was a beefed up, bigger version of the original, now with troop carrying capabilities. The next version had that POP up, snake shaped head...ok, this is by far my least favorite HISS. The DTC version used a different weapon system, a more believable armored canopy and a troop compartment. The PoC/30th version was and awesome new design, but still recognizable as the next in line of the HISS. The Retaliation/SDCC HISS was an improvement over the already awesome PoC/30th version with a glass cockpit and a manned turret.
Then there's the on screen HISSes of Pyramid of Darkness, Resolute, Renegades and Retaliation, all awesome in their own right.
So my favorite? While I definitely don't like the pop-up head version, I'd have to go with the Retaliation HISS by design. As a toy, it would have been greatly improved with the real working tracks of the PoC version. The version one and DTC are a close second.
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Some people, when trying to avoid thinking about those things that cause them pain, turn to drink, food, or drugs...I turn not to any of those...I collect action figures. Over the years, I've found that alcoholism is much more socially acceptable by far.