People love asking me this one, so I'll just answer it plainly: here's what I keep coming back to after years of watching what actually sells, and what actually gets complimented five years later.
Oval has been the reigning favourite for a while now, and for good reason. It stretches the finger, throws a huge amount of brilliance for its carat weight compared to a round, and it photographs beautifully. It's the shape I see requested most at every show I attend.
Cushion cut is my personal soft spot. Those rounded corners and that romantic, slightly vintage character give it warmth that a lot of the more angular shapes don't have. It pairs beautifully with a halo or on its own in a simple solitaire.
Emerald cut is the quiet luxury pick. It's a step cut, so instead of a lot of sparkle, you get these clean, elegant flashes of light, almost like looking into a hall of mirrors. It also shows off a diamond's clarity more than any other shape, so it rewards choosing a genuinely clean stone.
Worth watching: pear and radiant cuts have been climbing steadily, especially for people who want something a little less expected. My honest take, whichever shape catches your eye in person is the right one. I've seen customers talk themselves into the "popular" shape and regret it, and I've seen a pear-shaped ring stop someone in their tracks. Trust your own reaction.