Mount Gay Extra old is lovely (also known as special reserve I believe), the standard Mount Gay you get is Eclipse, it's ok but nothing special, Cockspur Old Gold is also worth a try, I lived off this stuff in Barbados where you can buy it in gallon bottles. Both of these variants may be quite hard to get hold of over here though, they tend to only import the cheaper versions unless you hunt around on line, unfortunatley my 1.5 litre bottles of both are almost empty. Easier to get hold of and very nice is Appletons VX, I think even Morrisons do it.
They are my favourites but I prefer gold rums, if you like white or navy runs then I can't really help.
Are you planning on drinking it neat, with ice or with a mixer? If you are planning on having it with a mixer I would recommend Ginger Ale, that's how I have mine.