In Dragon Ball Z , 17 referred to himself as the strongest fighter (before he knew the power of 16, Cell, etc). I don't remember where, but I'm positive that it was mentioned he was slightly stronger than 18. Now with the events of Dragon Ball Super and the Goku vs 17 sparring, is this still a valid assertion or has it been retconned?
Firstly, during the android saga, 16 was stronger than 17 who happened to be stronger than 18. Although the power difference between the 3 is certainly not a lot.
When Goku goes to recruit Android 17 to enter the tournament of power, it was stated by 17 that he spent that time gap training and had gotten a lot stronger. It's easily been over a decade or maybe even 2 since the cell games and while Android 18 hasn't really pushed herself and trained as much, 17 has spent his time training. We see 17 being able to fight SSJ2 Goku very easily and he also seemed to fair pretty well against SSJB Goku(Suppressed) very well. We know when Goku is not going all out because once he's forced to go all out, he immediately uses Kaioken just like he did with Toppo, Jiren, Hit etc. It was also stated by Goku that even Android 17 was holding back. So at the moment, Android 17 is high up on the ladder and very likely the fourth strongest fighter in Universe 7. Android 18, on the other hand, has received a relative power boost which all the characters, in general, have received to make them relevant in the tournament of power.
We see Piccolo easily defeat SSJ2 Gohan. Android 18 is very likely on the same level as Piccolo or maybe slightly stronger or weaker. Android 17 on the other hand is definitely stronger than SSJG Goku but not stronger than SSJB Goku.
17 and 18 have all the same abilities and unlimited energy, but I feel like 17 is probably a lot stronger, because apparently 17 continued training and seems to be close to Goku's level, while 18 became a mother and didn't really continue her training. I may be wrong, but as far as I can tell, this seems to be the case.
Android 17 is stronger than 18 because ever since he returned for Dragon Ball Super, he has shown everyone how strong he has become. Even his Barrier improve since we last saw it in Dragon Ball Z. Because in the tournament he used his Barrier in different ways like using it as a Barrier punch on Aniraza and making his Barrier spinning around to toss over that cat girl. It’s amazing how strong he has become, and to me he’s the 3rd strongest character in Universe 7. Since 17 would definitely beat Freiza in a fight.