Strongest, but for a beginner.
Okay. You will probably see a lot of votes for Inca and Maori. They are indeed ridiculously powerful, but I personally wouldn't suggest them right away for a beginner - you need a good understanding of what you are doing when playing as them (though I'm starting to suspect that Inca might be foolproof).
Since you are completely new to the Civ franchise, I'll probably just follow my own logic when I first got introduced to it:
1. I want science, because science is awesome and I get to crush the opposition with technological advantage.
2. I want a powerful military, because it only makes sense to expand aggressively until the world starts to notice.
3. I want something easy to play as. I have no idea of the mechanics, but I want to do well while gaining more knowledge.
4. Wonders seem fun and important (
NOTE: not really), I want them.
So, my suggestions:
Aztec. Very powerful early military with snowball potential. The ability to capture Builders and then use them to speed up district construction means that you will have an easier time handling your infrastructure. You just need to be aggressive.
Sumeria. Similar to Aztecs, though less forgiving in terms of keeping an eye on your city development. But the War Carts are ridiculously strong, so very early conquering will be easy.
Korea. Lots of flat science from their unique district, and later it gets %-boosted by her leader ability. The civ kind of plays itself.... More peaceful and vulnerable in the early game than the previous two, but it won't matter once you are ahead in science.
Nubia. Somewhat similar to Aztecs, but they focus more on ranged units. Have passive bonus to district construction speed, that can get ridiculous if you manage to properly set up the Nubian Pyramid.
China - if you want as many early wonders as possible. But again, wonders are not really that big of a deal, and the greater appeal of China is using that extra Builder charge on something else, or rushing that one specific wonder that you want to build your strategy around.
Pick any of these to learn the basics, and from there just read through descriptions of each civ and pick what sounds more interesting to you. The First Look videos on Firaxis's Youtube channel can be of great help.
