Pretty much any cat / feline kit is a must-have, even though I already have plenty! Similarly any running kit, especially distance running (i.e. half-marathons and marathons). Strangely enough, as a dietitian I don't have a lot of food-related kits, mainly because I hate the emphasis on weight loss and measurement that many "diet" kits have - I'm a firm believer in Health at Every Size (and if you've seen my layout for the last week of the most recent survivor you'll understand why), while still eating healthy (most food kits seem to emphasize either dieting or what could be considered "unhealthy" foods - I need healthy foods without the emphasis on dieting, as we know from research that diets don't work).
Most of the travel kits out there don't meet my needs (I need kits for ancient Egypt, ancient Rome, "old" Europe, etc.) so although I have YEARS of travel photos to scrap, a lot of them haven't been yet! Similarly, most of the scouting kits don't meet my needs as I was a Girl GUIDE for many years, and most kits are for scouting not Guiding, and they are different!
Oh, and I pretty much buy any Canadian-themed kit that I can find, unless it gets things totally wrong (for example there is a Canadian kit at another digital scrapbooking store that has the Canadian flag TOTALLY wrong, so I refuse to buy the kit because of that. I mean, can you imagine buying a USA-themed kit if the American flag in it was wrong? No, right? So why should Canadians have to put up with an incorrect flag!)
I also tend to pick up any hunter/jumper or dressage riding / horse kit that I see, as too many horse-related kits are Western-themed, and I've always been a hunter/jumper or dressage rider.