A sharpie works if you bury the marked portion in the soil. Just leave a good end sticking up so it's easy to find the label later. For weatherproof that is visible all the time, the only thing I have found that holds up in our southern sun is label tape specifcally made for outdoors. Of course, you have to invest in a labelmaker, but I can say that I have thoroughly enjoyed mine for years. (I grow lots of daylilies so I use it alot.)

I use pencil.

paint pen
sharpie if coated with clear nail polish
grease pencil

Grease Pencil....also known as a China Marker

I still use a sharpie, but the labels I use are sections from a plastic cup. The cups I have are red with a white interior. I cut wedges and write on the inside and place them in the soil so the writing is shaded from the longest sun exposure. The writing lasts the season that way.

Another vote for grease pencil aka china marker.

Pencil.

Pencil...no more Sharpie for me...

Sharpie extreme are made for outdoors and construction