You specifically mentioned stock certificates, so I'll leave bond certificates for another time.

Here are a few of my nicer ones.
American Express Co. showing the geometric star cancel frequently misattributed as a postal cancel, certificate signed by Henry Wells and William Fargo of Wells Fargo fame.

Early stock certificate with R36a (CV $500).

Attractive vignettes.

Red and green is always a great color combo.

Very scare 25-cent Warehouse Receipt part perforate (R50b, CV = $1,100).

Very unusual, a completely holographic (handwritten) stock certificate.

Some others:



