What makes a icon great is usually judged by how fast a user would recognize what it does. The most common one would probably be the iconic floppy disc the indicates "save". No one would hesitate its meaning, even though tons of young users probably never seen an actual floppy disc or knows what it does. But the would understand it because we see it everywhere.
Other symbolic icons such as a trash can for "delete" and a "+" indicates "new". The print-icon is yet another one everybody instantly recognizes.
There are those I find a bit more problematic in terms of intuitively understanding its purpose and meaning. "Download" is usually visualised with some kind of downwards facing arrow. There are variations but I tend to misunderstand them more often than what I would like. One issue I have with it is that it is extremely generic. Arrows are everywhere, from your keyboard's directional arrows to the browsers and apps navigational back-arrows. An arrow doesn't in any what stand out. While browsing the apps on my phone (which is an Android one), I honestly struggle to find the "downloads" app just about every time. Yes, it is indeed presented with the downward arrow.
Another problematic one is "share". Every single website have its own little idea on what sharing should look like. The one I see most is the the three-connected-dots. It a general share icon on the Android platform. Apple has gone a different way with its arrow-out-from-a-box. All the big social platforms have their own symbolism to play with, so users does not have a shared symbol to look for. This annoys me, as this is an extremely common action that you can find everywhere. I think we deserve an icon for "share" that is just as common as the floppy disc.
Perhaps a good idea would be for me to suggest new ideas for these icons. But the thing is I don't have any. Not a single idea. "Download" is really hard to express visually. Its kind of a virtual action with no good resemble to anything else. "Sharing" has the problem of being to common so the icon sits in the idiom of platforms rather than the one of icons.