Angreeley: Every printing process has its fan and detractor. How do you measure quality; much is subjective. Engraved stamps come top of my list (no surprise there), for the skills employed at all stages of production (yes, even when computer engraved) and are second to none.
The design should always be applicable to the process used and many designers, especially from the "computer age", do not seem to appreciate this! Some gravure stamps, for example, should never have used that process, as the design does not take advantage of the tonal qualities of gravure. Similarly, some engraved stamps printed by intaglio offer nothing by using that process (as Rein states) and could have been done by solely using offset, which would be much cheaper and have no loss in visual quality or impact. Some administrations use offset instead of using intaglio to the total detriment of the finished stamp, which often appear flat and lifeless.
Each process has specific attributes and I have made an attempt at describing some of these below.
Intaglio: A high security process (hence its continued use on banknotes). Difficult and expensive to replicate by forgers. Great for all line work, portraits and vignettes. Very tactile with its raised surface. Works especially well when printed in just one colour, or at most two (a personal view!).
Letterpress: Currently undergoing a resurgence, but not on stamps. Older letterpress stamps could invariably be best identified from the reverse side because the design often felt raised to the touch due to the use of the metal plates and heavy pressure used. These days, plastic plates are used and tend not to leave the raised reverse. Modern "Letterpress" stamps tend to actually be by flexography and are indistinguishable from offset to the eye.
Gravure: Especially good when designs utilise tonal ranges (i.e. gradations of colour). Best suited for long print runs (some printers say 10 million plus stamps), so do not expect a tiny island that posts very little mail to print by gravure, unless they have a huge print budget!
Offset: Best suited for small to medium sized print runs. The widespread use of offset saw multi-coloured stamps rapidly take-over from one or two coloured stamps. This is the most readily accessible form of commercial printing available today, with comparatively low set-up costs and suitability for runs up to around ten million stamps. Today, around nine out of ten stamps that are produced worldwide use offset. (Source of usage statistic: The UPU, Switzerland.)
Silk screen: Great for where heavy ink weights are required, e.g. the latex 'scratch-off' areas on some stamps, or large areas of solid ink colour, such as a mono-coloured background. Not good for, say, 'fine line' text.
Digital: Currently best suited for small print runs (but slowly increasing in volume as new machines develop). Every stamp on an entire print run could theoretically be different (i.e. unique). Easier to forge, as many of us own some of the production equipment in our homes, such as colour laser printers. There are not enough digital stamps "out there" yet for forgery to have taken hold, but I suggest that it will become a problem over time.
As for thematic (topical) collectors, I used to collect based purely on design and the ability of that stamp to advance the story that I was attempting to tell. I cannot imagine that any other thematic collector would buy a stamp unless it advanced their story, which implies that the stamp had the "detail and quality" that he sought. The process used would not enter into my buying decision, nor would poor quality printing if the stamp depicted the only known example of an 1852 letter box from a particular country on a stamp!
I hope this may help. GLENN