British Guiana

If you can financially see your way past the first 1850-1851 “cotton reels”, and of course the world’s rarest stamp, the 1 penny magenta (which recently sold for over $10 million), then you can build yourself a nice collection, though it must be noted that it is more difficult to do mint.  Mint Guiana from 1852-78 in particular represents somewhat of a challenge, given that in most cases the overwhelming number of stamps issued were used postally.  Guiana from the turn of the century is easy to do mint.