What do dog and cat owners like to buy the most?

My guess: pet food.

funny little outfits for their pets

pinkfreud's comment is certainly right on target... but owners
definitely like/enjoy buying toys and stuffies to keep their pets
entertained and stimulated. I know I get a kick out of watching my 3
dogs play with their toys and cuddle with their soft stuffies.  :)

Best regards,
tlspiegel

Dog owners in my experience mostly buy small breeds of dog that are
quiet and not to messy.

Found a cool gift idea for pet owners at
http://www.lucy-the-talking-bird.com.  They will create your own
personalized eBook with your pets as the characters.

Hard to answer as the question seems a bit ambigious to me. I'll try
both interpretations.

1) If defining "the most" in terms of dollars, the answer is
unquestionably dog food, as this is the main expenditure.

2) If defining "the most" in terms of giving the most pleasure, I
would guess toys or treats. However, this is just a guess. In some
places (Paris, New York), various accessories (in particular clothes)
are very popular.

=================

Elaborating on point 1, I would note that dog food owners are moving
away from ECONOMY dog foods into increasingly specialised dog foods
(Premium, Natural, Holistic, BARF, etc.). For a review of the options
and the owner's motivations, this resource may be of interest to you:
http://www.dog-food-corner.com