I've been on/am on a few forums that do a COTM/ROTM. There are times where it seems it is more of a "popularity contest" but for the most part, the nicest car usually wins. On the 3/S forum I am most active on now we actually have categories for each month. Example 500+ hp only (had to be proven with a dyno sheet), Stealth only, 3000GT only, any 3/S/GTO (GTO is what the 3000GT is called in Japan and some other markets), N/A only, TT only, etc.

The categories work out well because it evens out the competition (for the most part) from month to month. If you wanted to do something like that you could break it into categories like all w body, 5th gen, 6th gen, 7th gen, NA, SCed, etc.