My vote is for C and D cell incandescent Maglites. They are cheap and their aluminum body and head parts are fairly well made and durable, so they can handle a lot of abuse. They are also, hands down, the most popular flashlights for modifications so off the shelf upgrade parts are widely available as is advice about just about any kind of modification that one can dream up. There are also countless articles out there from previous modders with detailed step by step instructions on what they did to their Maglites.
Oh, and this is the real kicker, C and D cell incandescent Maglites are dirt cheap! You can hack away at them without worrying about messing up because for just a few more bucks and a trip to just about any hardware or general store, you can get another brand new one to work on again.