For me its my 1/4" drive metric semi deep socket set. When I first saw semi deeps I thought they were pointless since shallow or deep should cover everything a semi deep would cover, right? I ended up getting my semi deeps dirt cheap as part of a deal on some other stuff I was buying, and they quickly became my go to for 1/4" drive as they are just right for everything. I never even use my shallow or deep 1/4 drive sockets anymore, just the semi deeps and flex sockets.