There is no best attachment. There is only the one that matches how the cover will actually be reloaded, and by whom.

Corner slits

Fastest and cheapest, tool-free. Loads the sheet at four points.

Tears the paper at a corner first, especially on uncoated stock.

Top-load pockets

Spreads the load along a whole edge, so the sheet lasts far longer.

Slower to reload, and the window is then the wear part.

Screw posts

The only mechanism that takes a change in page count.

Gets stripped by a hurried reload. Buy spares and a coin-turn head.

Elastic cords

Gentlest on the paper: nothing punctures or grips the sheet.

Relaxes with cycles and heat until it will not hold one thin sheet.

So ask one question

Who reloads it, and how often? Everything else follows from the answer.