Most revenue collectors that I know are generalists, trying to obtain one of every revenue stamp listed by Van Dam, without getting into the different varieties that exist. However, there are a number of advantages to be gained by choosing an issue and specializing, the main one being the possibility and thrill of new finds. The possibility is very high in this field, and it is one that provided you stick to a topic over the long term, you are bound to make money on when you sell. The reason is that revenue stamps are much less expensive for what you are buying than postage stamps are.
Another advantage to collecting them in parallel with your favourite postage issue is that you can use what you learn from your study of the revenues to help in authenticating some of your postage stamps. For instance, if you collect Small or Large Queens and you also collect the 1868 Federal Bill stamps, you can learn for very little money what all the Duckworth papers and gum types are supposed to look like and then you can apply that knowledge to the your study of the Small Queens. For example, take the 1c block above. This is what the gum side looks like:
That gum is dull and streaky. If you read about the gum on the Small Queens you will quickly learn that for the mid-Montreal period between say 1874 and 1880, the gum tended to have this appearance. It most commonly associated with the Montreal printings perforated 11.5 x 12. So strong is this association, that when I saw this block, I was 99% certain that it would be perf. 11.5 x 12 when I measured it. Sure enough, it is indeed perf. 11.5 x 12. So I know from this, that this block is from between 1874 and about 1880. Now whenever I see a mint perf. 11.5 x 12 Small Queen, I can compare the gum to this to see if it looks original, which is very useful.
One disadvantage to revenues is that for many sets the stamps are all the same basic colour, with just varieties of shades, which if you like a lot of colour, limits what you can collect. Although even the monocoloured issues do often exist with trial colour proofs that you can collect as well. Another disadvantage is that large accumulations of revenues are hard to find and consequently, if you want to specialize you will find yourself having to be more patient in acquiring material. You will probably buy most of your stamps at retail as opposed to auction because most revenue stamps are not valuable enough for auctions to offer them separately. Finally, in terms of condition, it is much harder to find pristine condition. The main reason for this is that they were used on documents that often got folded through the stamps, or stapled of punched. In short, they were handled with much less care than postage stamps. However, pristine stamps do exist, for those with the patience to seek them out.
If you are looking to specialize as opposed to forming a general collection and you like beautiful classic engraved stamps then your best bet is to choose a series that has a long period of use and for the category of revenue is one that would have seen a lot of use. My top picks for these sets are:
1. 1868 Third Bill Issue as shown above in use for over 20 years.
2. 1915-1928 Two Leaf Excise tax
3. 1934-1948 Three Leaf Excise Tax
4. 1864-1870 Ontario Law as shown above
5. 1870-1911 Ontario Law - lots of shades, paper and perfs.
6. 1929-1940 Ontario Law - less perfs but still lots of shades.
7. 1871-1890 and 1893-1906 Quebec Law
8. 1870-1912 Quebec Registration - the Beaver design. - lots of shades, papers and perfs.
9. 1908-1968 Saskatchewan Law - just shades, papers and gum varieties, but lots.
10. 1932-1948 Postal Note and Postal Scrip - lots of beautiful colours, papers and gums.
Which one of these is best for you will depend on your budget, whether having lots of colour is important to you, and whether you like to collect primarily shade varieties, paper varieties, gum varieties or perforation varieties.
If you can spend $500-600 on your most expensive stamp and you like lots of varieties, but you don't need to have a lot of different coloured stamps, then without question, I would say the 1870-1911 Ontario Law stamps are your best bet. They span a 41 year period, there are tons of perfs and papers, and finally the shades on the stamps go from yellow to orange, to red, to brown red and beyond. If you can spend over $10,000 on you most expensive item then I would recommend the Third Bill Issue. I can still recommend that to a collector of even modest means as long as they don't mind not obtaining the rare inverted centre on the $2. If having different coloured stamps is important, but you still like lots of varieties, then the Quebec Registration stamps are your best bet.
If on the other hand you really don't want to fuss around with perforations and paper varieties and want to focus instead on colours, then either the 1929-1940 Ontario Law, or the Saskatchewan Law stamps are best, as there is only one perf and most of the variation is in the shades.