@lunabee Generally: at least FFP2, N95, KN95, or whatever your local equivalent is (it depends on where you live). There are higher certifications (like FFP3) which do filter better, but they can be significantly more expensive and don't necessarily make a big difference for covid.

If in EU, look for FFP2 - verify that there is a CE logo with a 4-digit code printed *on the mask*; if it's only on the box/packaging or the digit code is missing, it's not genuine.

I have good experiences with the 3M Aura 9320+, they seem to provide a good fit for almost everyone including me, and they have good nose clips, though they are not the cheapest. Be careful with folding masks with ear loops, they often have a poor fit and the ear loops can cause ear pain (I strongly prefer headbands) - you must have an airtight fit for the mask to work as advertised, and different masks fit different face shapes.

Beware of masks labelled CE 2163; there are questions about that specific certification company and whether they actually tested the masks correctly, and I don't think a clear answer to that ever surfaced. Most cheap masks off places like Amazon are certified by this company.

Any mask that has a valve or opening is unsuitable, basically. The famous blue rectangles will protect others from you to a limited degree, but will not protect you; they are not form-fitting.

I've also seen a lot of people recommend full-face respirators, and it makes sense to me that those can provide a better fit (and would therefore protect better), but I do not have personal experience with them for practicality reasons.