Considering that Toriyama has been quoted as saying Krillin's the strongest human a couple times, and the guidebooks also say so, I guess Krillin is probably the strongest... at least during the parts of the timeline where those quotes were made in reference to. If Tenshinhan isn't the strongest then he's the obvious pick for #2, since he was always the strongest of the humans before Krillin's "potential unlock" happened and he's been training non-stop (including a year on North Kaio's planet) ever since then... so he shouldn't be
too far off from Krillin's level. Yamcha seems to be the straggler out of the main three humans, aside from filler and video games randomly deciding to boost him every now and then... but those don't really count.
So something like...
0: Uub (obviously. XD But if you're not counting Uub, then...)
1: Krillin (at least as of the Buu saga and Resurrection F. It's possible that Tenshinhan will surpass him over the years if he doesn't keep up his training.)
2: Tenshinhan (as said above, he could surpass Krillin in the later years depending on Krillin's training habits, but as of Resurrection F, Krillin is apparently still the stronger of the two.)
3: Yamcha or Chiaotzu (I think it's possible that the albino midget's managed to surpass Yamcha by training with Tenshinhan so much while Yamcha's slacked off after Cell. Krillin at least was still interested in tournaments as of the Buu saga even though he knew he wasn't going to win with the Saiyans, 18, and Piccolo competing, so it seems Yamcha's level of slacking is far beyond Krillin's -- he may have even gotten
weaker due to just not ever using his power over the years after Cell.)
4: Whichever of the above two wasn't #3.
5: Yajirobe or Roshi (Resurrection F has Roshi fighting and defeating Freeza's soldiers... they could just be really weak soldiers of course, or maybe Roshi's randomly pushing close to the 1000 mark now, which would put him at or above Yajirobe's last known level... it's hard to say which. Assuming Yajirobe hasn't been training since the pre-Nappa/Vegeta year, anyway; if he has, he's definitely #5.)
6: Whichever of the above two wasn't #5.
If you read the manga, then you'd naturally believe Kuririn is the strongest human... considering Yamcha SAYS so. Let's be real here, everyone would except the people who want Tenshinhan to be #1.
Because a guy who probably hasn't seen Krillin in 7 years, definitely hasn't seen Tenshinhan in 7 years, and is currently trying to reassure Krillin's daughter that his dad will be okay fighting in a martial arts tournament, is
totally an expert on exactly how strong the two are in relation to each other and couldn't
possibly be fudging things a little for the sake of not making Dad look bad in front of the kid.
That said, it's entirely possible Krillin still has a lead on Tenshinhan in terms of raw power, thus making him the strongest human fighter -- and that's what statements by Toriyama and the guidebooks are saying, too. But it's really, really dumb to take some off-hand statement by Yamcha of all people as some kind of absolutely true statement of power comparison, especially considering the context.