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)