Ummm hmm.. this is a good one.. for Thumper, I'd have to pick a few, since he was my first and only dog I've had from puppyhood, and hes an australian kelpie, australian shepherd and border collie mix (smart/active, etc) .. a few of the best purchases I've made for him were crates(s) (a smaller, size appropriate folding wire crate for the bedroom and for travelling), and a very large folding 4'x4'x4' metal pen with a roof that pretty much stays permanentley in the corner of my kitchen which he goes in during the days. Of course, he's pretty much weaned off of both crates now as he sleeps in the bed at night, and gets full roam of the house occasionally, for long hours he still goes in his large kennel.. but these have definitely been lifesavers when he was just a baby. The treadmill I bought him was definitely one of the best purchases, for winter times here when its too cold to go outside, he exercises on his treadmill and he absolutely loves it!
The other few for Thumper that have been extremely helpful, is a floatable, non-destructive rubber ball and 'chuck-it' and a rubber frisbee , to tire him out immensely at the dog parks, and a harness because he runs and pulls us on rollerblades in the summer and something else he extremely enjoys and gets a ton of exercise while doing it lol.
My Shiba... hmmm.. well not sure what the best purchase I ever made for him, he never required a crate, although he has one in the bedroom which he chooses to sleep in occasionally, and i've never really bought him training devices or toys because he never needed them or had interest in toys.. he himself was probably the best $1500 i've ever spent, he was an expensive dog but worth every penny one of the best dogs you could ask for.
A water bottle with holder off of Etsy. I had to wash and refill her water bowl 5 times a day or she wouldn't drink out of it because she'd gotten some food crumbs in it. Now with the water bottle I don't have to worry about that.
I know not in the true spirit of the OP question but the best purchase I ever made for Jubel was his adoption fee. Sure he loves the toys I've bought when he first sees them but then they are either destroyed or boring soon after. He loves the food/treats/chews but he's a hound who will eat anything. He enjoys the classes we take and the time he spends at daycare but he's always happy to be doing something with me or playing with his doggie buddies.
If I had to pick something it'd have to be the food dispensing toys and those are repeated purchases as they only last so long before he manages to break them and I like to try new ones as well.
I don't know if I could think of a "best" purchase, but I can think of some that have been really really good, useful or fun....
Ruffwear Webmaster harness-- so versatile, durable and comfortable for Chester. He wears it for car rides (carabiner attachment to child seat latch), hiking (he can pull uphill without hurting himself and I can hold him close for narrow areas etc), tie-out (supervised of course but far safer than a collar), crowds, playing on the 50 ft leash (safety and freedom for him)
50 foot leash- I don't use it that much but when i do, it is so very useful. Rural areas, quiet parks and my parent's unfenced yard and he can have freedom to roam and run while being secure and visibly leashed (its a blaze orange nylon line aka "best dog training leash")
High quality dog food- the difference in his coat, skin, poops and overall health is obvious on a high quality food compared to Science Diet (from the shelter) or Purina. The cost difference is not actually that much, only about $15/month extra since I feed less cups per day as the ingredients are all more digestible and provide more suitable nutrition
Training- classes or self-training or both, but time and effort spent training is paid back multiplied when you end up with a dog that can go anywhere, deal with anything and loves everyone (obviously everyone's dog is different, but even if a dog has issues or difficulties, anything that improves their behavior and confidence is a good thing)
A 5 in 1 shot. The lady that had Saint felt that we were a great match and that she would give to me under the condition I bought her the shot and administered it to Saint in front of her. Best $7.09 I've ever spent!
Some of the best times we have are blasting across the bay at 50 mph ...My dogs love being on the boat and jumping in the water and swimming..catching fish and running across the sand with wet fur.
Besides a training course I'd say her cheap stainless steel food & water bowls. I don't know why, but I think she prefers to eat from either the ground or metal. She ends up moving food from paper plates or plastic to the ground before eating it.