Apologies to the non-wargamers on the forum but I wondered which rules people like and what attracts them to a certain set of rules. For me it would be (in no particular order of preference)
Crossfire - I love the slow burn as you gradually get your troops into their starting positions without taking risks and then the constant calculation of risk as the action unfolds which involves you really trying to look at the ground from an infantryman's point of view. Where is the dead ground? I might attract fire but will I at least have some cover? Can I use smoke to shield the next move? Have I now got to a position where I can risk a big push? I think they really give the feel of small unit combat.
Lion Rampant - They are just so playable but still manage to give enough of a period feel - not super realistic and perhaps not for the purist but give a great game. Just the right amount of what we call the "buggerance" factor to test the quality and flexibility of a plan, especially when your expert archers decide that they'd rather stand around chatting instead of firing at the shedload of knights thundering towards them! (not that I'm in any way bitter about the 6 points I spent on the bunch of layabouts!)
Homegrown Dark Age rules that grew out off the Command and Colours boardgame system - I just like big sweaty blokes in mail with dangerous choppers. Seriously, it's all about the specialist dice with symbols that allows you to lower or reduce the likelihood of casualties without having to use shedloads of dice and the movement system that punishes breaking up your army into small bits that makes it harder and harder to manoeuvre.
There are lots of other rules I like but these are my favourites, on the other end of the scale is Saga, I know lots of people like them but I never felt like I was actually playing a wargame and it is was more about understanding the battleboard than the capabilities and tactics of the historical troops your are using.