Good of Bad Stamps

Aside from any specific collecting goals that dictate the particular stamps you are looking for, there are some general guidelines that apply to any used stamps you buy. I think to qualify as a "good" stamp, you would look for these qualities:
1. A cancellation that is not too heavy, and preferably smudge free. Also, on any stamps picturing their majesties, many collectors prefer cancellations that do not cover the face.
2. A clean stamp (no dirt or ink stains from colored envelopes), with no tears and no missing perfs (yes, technically, a perf is a hole, but it is perfectly acceptable to refer to the bits of paper between the half-holes as perfs).
3. No bends or creases in the stamp.
4. Decent centering. Most "good" stamps should have designs that are well clear of any edge, and should not have greatly off-balance margins.
5. No toning. Many stamps will show brown spots on them which can be from any number of causes, for example an acidic chemical reaction from the gum or the page it was mounted on, or the stamp hinge itself. Some spots might be a result of foxing, which can be due to mildew or, again, chemical reactions within the stamp such as the oxidation of minerals within the paper or the inks.

That's just a few things I can think of off the top of my pointed little head. Someone else might chime in with more points. But I think if you keep these in mind, when choosing your stamps, you will wind up with an album full of good stamps.