Like all game genres, RPGs have role models - other games they look to for inspiration.
The Zelda series started out back in the days of the Nintendo Entertainment System, when it and the original Super Mario Bros became the most recognised NES games, and have evolved to be some of the biggest video game series there are.
Fast forward into the days of the Nintendo 64, The Legend of Zelda Ocarina of Time was released, and it instantly became a hit. Annoying pixie aside, OOT is now remembered by a lot of gamers as their favourite childhood game.
On the PC side of things, the Baldur's Gate series has always been held up as the father of modern PC RPGs. With its Dungeons and Dragons-inspired style of play, it captivated millions of gamers who spent many hours in front of their screens clicking away.
The series' little kids live on today in the form of Dragon Age Origins, and its yet unreleased sequel coming out soon, both developed by Bioware, the same company that made Baldur's Gate and its sequel.
The Elder Scrolls 4: Oblivion
Considered by many to be Bethesda's best game, Oblivion is praised for having great gameplay and, at the time of its release, unmatched graphics. Backed up by hundreds of hours' worth of just plain stuff to do, Oblivion was a huge success.
Fallout 3, another popular RPG developed by Bethesda, has been criticized for being too much like Oblivion. I suppose the developers figured that if it's not broken, don't fix it, which just goes to show what an impact it had.