Excluding Zeno because he is the king of everything, who is the strongest character in each Dragon Ball series, like Dragon Ball Super, Dragon Ball Universe, Dragon Ball GT, ...?
I'm looking for fact-based answers, like using logical based such as comparing Ki and lot more.
Each of the series introduces multiple characters of increasing strengths during different arcs so I'm going to answer the question with the strongest character at the end of each series.
Dragon Ball: The strongest character at the end of Dragon Ball was Goku. Piccolo was the next character along with Kami in the race. However, Piccolo addressed Goku as being the only real obstacle that stood in his way and that Kami had neglected too much on his training.
Reasoning: In Episode 148, we see Piccolo and Goku fight where the both of them seemed to be pretty even in strength until Goku finally rung him out. Also, Picollo had wounded Goku to such an extent that he was in a life or death situation and it is obvious that he would've gotten a Zenkai boost after their fight. The only other fighter in the race is Kami but because of Piccolo considering only Goku to be a threat, I think it is fair to assume that he was the strongest.
Dragon Ball Z: Dragon Ball Z saw the introduction of fusions. So the strongest character including fusions would be Vegito which is a Potara fusion between Goku and Vegeta. The strongest unfused character, on the other hand, would be, Ultimate/Mystic Gohan. The other characters in the race would be Super Buu(Who was even stronger with Gohan, Gotenks absorbed), Goku and Gotenks who weren't as strong.
Reasoning: The final antagonist at the end of Dragon Ball Z was Majin Buu and we see multiple iterations of Majin Buu. We know the SSJ3 transformation is a lot superior to the SSJ2 transformation. Majin Buu in his fat form was slightly weaker than Goku in his SSJ3 form who was initially the strongest at the beginning of the series. Then we see Super Buu(The strongest iteration of Buu), who was about the same level as strength as SSJ3 Gotenks (Who was a fused character). Fusions are more powerful. Gohan in his Ultimate Form, was strong enough to Toy around with Super Buu in episode 267. Gohan was also able to hold his own against a Buu with an absorbed SSJ3 character in the following episodes. Finally, we see Potara fused character Vegito, completely Toy around with a Buu who absorbed the strongest unfused character Ultimate Gohan, making him the strongest.
Dragon Ball GT: "Note: This series isn't canon". However, this series also contained fusions hence the strongest character at the end of the series was SSJ4 Gogeta, which is a fusion between Goku and Vegeta in their Super Saiyan 4 transformations. However, the strongest unfused character would be the last villain they had to face, that was Omega Shenron.
Reasoning: This one is fairly easy. Omega Sheron was the last Antagonist of the series. He was strong enough to push back SSJ4 Goku, Vegeta, Majuub, Gohan, Goten and Trunks.It was only after Goku and Vegeta fused into Gogeta, they managed to completely Toy around with Omega Sheron. The last few episodes of the series between 57-63 would indicate the same.
Dragon Ball Super: So this show saw the introduction of Gods/ Angels and took power scaling to a multiversal level. The strongest character in terms of power is the Grand Priest. "Note: The Omni King doesn't have physical strength as stated by Beerus and Whis. He has the ability to erase anything from existence solely out of will which makes him powerful". If I were to consider only mortal's, the strongest character currently is yet to be determined, however will be soon in the next few days between Goku and Jiren.
Reasoning: Unlike in the previous series, this series introduced many characters stronger than the main characters by huge margins, without actually revealing their power in actual combat. We got introduced to Gods of destruction and Angels. Gods of destruction are technically the strongest fighters in every Universe(With the exception of Jiren and maybe even Goku now as of episode 129).
There also exist Angels who are substantially stronger than the Gods of destructions, to a degree that they could take them out without any effort. One such Angel who is friends with the protagonist is Whis. Whis said to Goku in Episode 55 that even he could barely keep up with the Grand Priest in terms of combat. Note Whis himself at that time is strong enough to knock out Goku and even the God of Destruction of his universe Beerus(Who is substantially stronger than Goku at that time) without even trying.
As for the strongest mortal, After Episode 129 and 130, it has to be Goku in his Fully Mastered Ultra Instinct Form with Jiren not Far behind!
