Well, It's hard to choose just one, when there are SO many to choose from, so here are five I would choose between, the effects, how much they cost, and where they can be brought or found:
Joy Ribbon
Causes a gradual experience increase 5,555
Kecleon Store
Random prizes
Random dungeons
Heal Ribbon
Heals HP gradually 3,500
Kecleon Store
Random prizes
Random dungeons
X-Ray Specs
Enables Pokemon to see location of items and foes 6,000
Kecleon Store
Random prizes
Random dungeons
Bright Specs
Allows for Traps to be visible 5,000
Kecleon Store
Random prizes
Random dungeons
Raise Orb
Your HP becomes the same as your partner with the highest HP 300
Kecleon Store
Random prizes
Random dungeons
Obsessively this is just my opinion, and so you might disagree with my answer, but I hope I was at least able to narrow down your choices and make it easier for you. I hope I helped you!