'Discovered by' tag logic

"Discovered by" is on a per-body basis, not a per-system basis. You must use a detailed surface scanner on an individual celestial body, and be the first to sell the data related to that body, in order to receive credit. You do not receive any credit for just using a discovery scanner to receive the "undiscovered" markers, nor do you get any credit for scans obtained without a detailed surface scanner.

This has changed since the question was originally asked, and exploration in general has become more profitable.

Using the Discovery Scanner (aka "honking") a system reveals all bodies in a system in an "Undiscovered" state. This gets you rough data on the locations of bodies in the system for a decent reward if doing it a lot. Then you can travel to the bodies one by one and target them at some distance to get additional Discovery data (and credits), alternatively use the Full Spectrum System Scanner from any location within the system (given you have a line of sight) to achieve the same but at a much faster rate. When selling this surface data you can get additional efficiency bonuses, and the "First discovered by" tag only if you were the first one to do so.

If you have the optional Detailed Surface Scanner module installed mapping out each ring and body surface gives a lot more credits, compared to just honking and targeting or using the FSS scanner. If you were the first to do this for a body you also get the "First mapped by" tag when selling the data.

Even more data can be gathered by landing on some planets and exploring with the SRV, If you have Horizons.