I think the one from Halo 3 is the smallest, and would be the best scale-wise:

http://haft.bungie.org/images/RvwGreens.jpg

5 inch figure next to a 4 inch Joe would look pretty good, I think.

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Sweet! How is the articulation on it?

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The articulation on the current Halo figures is restricted heavily by the armor, so I'm guessing the same can be said for this guy. Some pics here to give you an idea how much he can move around, plus a scale pic with a Stormtrooper. http://www.mwctoys.com/REVIEW_030508a.htm

Since most Star Wars figures are about a head shorter than Joe I think it would work decently.

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Yeah the original Halo 3 figures should be in pretty good scale with modern Joes. Master Chief is supposed to be around 7 feet tall or something and the 5" figure, while a little taller than it should be to be in scale actually works well.

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