I also enjoyed both the Witcher series and Elder Scrolls series. But they are very different. Personally, I like the Witcher series better because it has a really strong sense of place and tone. Story and characters are very important. The first game is noticeably rougher than the later entries but I still liked it a lot. I actually haven't played the third one yet because, as Sarisio mentioned, it requires a beast of a PC, but everyone seems to love it and I'm looking forward to it once I'm able to upgrade my hardware. I highly recommend the series, especially if you're interested in good stories with interesting characters and tough choices, steeped in some excellent atmosphere. I also personally prefer playing in Polish with English subtitles, as I think the English acting is worse. Easy to do, with the versions provided by GOG (actually slightly harder to do with Witcher 3 for some reason, but still possible).
The Elder Scrolls series is an interesting one to follow. I've never played the first game Arena, but I've played all the rest. They've always had a focus on an open world where you're free to wander and do whatever you want, including ignore the main storyline. Over time they've been de-emphasizing stats and numbers in the interest of a "streamlined" experience, which some like because they feel more immersed in the world, but others dislike. There's also a valid criticism that while the recent entries have beautiful worlds to explore, there's not that much to do in them except enter yet another (mostly linear) dungeon and fight things, with combat that isn't even that interesting or fun. But I still had a lot of fun with the series including the recent ones.
Also, ever since Morrowind the games have absolutely phenomenal mod support, with mods available that do everything from tweaking little gameplay details to creating entirely new games. These really expand the games beyond the base that Bethesda provides and allow players to enjoy the games for years. The games also aren't strongly connected to one another, so it's easy to jump in at any point.
If your'e interested, here are a few thoughts on the Elder Scrolls games I've played:
Daggerfall: Used procedural generation to create an ENORMOUS world. Has particular nostalgia for me. I still think it's really interesting, but valid criticisms include the fact that the hundreds of towns don't really feel that different, and there's not much to do other than explore huge, labyrinthine dungeons pieced together from various building blocks. Also introduced (I think) the skill-based system for leveling, but there are a huge number of skills, many not that useful, and the system is fiddly. I still love it.
Morrowind: The most interesting world. Hand-crafted this time, with giant mushrooms, buildings hollowed out of huge crab carcasses, and other crazy stuff. Even the fact that it has the worst combat of the series and much of the map is a volcanic wasteland does not detract from its charm. Many players' favorite entry. Also, TONS of mods available.
Oblivion: Hasn't aged that well. At the time it had amazing graphics and one of the first truly convincing forests in a first-person game, but now the visuals aren't so impressive, and the world design is much less interesting than that in Morrowind. Also, bad voice acting with the same few voice actors across the whole game. But it has better combat than Morrowind (still not amazing), and magic feels better even though there are fewer types of spells to cast. There's more of an emphasis on following quest lines, either the main story or separate quest lines for each guild (Fighter's Guild, Mage's Guild, Thieve's Guild, etc.) Mods can make it more interesting, and the Shivering Isles expansion is much more interesting to explore.
Skyrim: More of an evolution of Oblivion, but I liked it better. Combat hasn't really changed much and is starting to feel stale at this point, but a beautiful world, slightly better voice acting, and a bigger emphasis on just exploring rather than following quest lines. Again, mods can add a lot. I actually haven't even tried the main story in this one yet!
OK that was probably way too much information, so I'll stop there.