I think the larger (C or D cell) MagLites are the most mod friendly lights followed by the Surefire 6P platform. Since you can get 2C/D up to 6C/D models you can get longer or shorter hosts to start with and their larger diameter bodies lend themselves to fitting different battery combinations. Replacement heads, tails, bezels, lenses, reflectors, and switches are available aftermarket (you can build a custom Mag where the only part that came from Mag Industries is the body). The larger form factor compared to the 6P allows you far greater possibilities as far as mods and possible custom work that you can do to one is concerned. The 6P, however, is far more carryable making it more versatile as far as the end product is concerned.