Nearly ready to bottle. Best priming agent?

The kit is probably corn sugar. Corn or cane sugar are the easiest to measure and most predicatable options.

 

If you are exploring priming alternatives, you will probably benefit from this calculator...

http://www.northernbrewer.com/priming-sugar-calculator/

 

Your kit probably came with 5 oz of corn sugar. I usually use this tool for determining how much sugar to use to carbonate my beers:

http://hbd.org/cgi-bin/recipator/recipator/carbonation.html?10386693

Like atpca said, corn and cane sugar are predictable and give consistent results. You could try using dry malt extract instead. The small amount you use won't really affect the flavor, but I feel it gives a slightly softer (not lower) carbonation.

 

Hold in mind that the small amount of whatever form of saccharine that you choose for this will have very little effect on the flavor of the beer. Of all the parts of brewing that could be considered artistic, this is probably the least of them--a utilitarian means to an end.
Use a sugar that will provide the yeast a fairly easy pathway to that end, particularly if your strong ale was particularly strong. Corn sugar. Easy to measure and easy to predict the result, as atpca said.

 

on the other hand, if that "fine white powder" is cocaine, it becomes a very expensive beer when you add it.

 

Flat, but peppy.

 

In order to get some flavor from your priming sugar you'd need to use something with some non-fermentables in it. Otherwise, it just ferments out. From those choices you are looking at molasses, dark brown sugar, maybe honey. But these might not get you the caramel you might be looking for. And you'd probably need to use more than the carbonation level calls for to retain the flavor.

My advice is to treat priming as priming and get your caramel flavor from the brewing itself at the recipe or process level next time you brew.