It's hard to say as it depends on your network and also subject to environmental differences such as weather pattern...
There's a black spot in a road signal wise where I drive each day on the way to the beach here and I judge my phones on weather they can sustain a call in that black spot or if they drop the call...
However in 2014 pretty much all the flagships I had managed to sustain a call passing through that black spot area.
I'm sure there's some official metric that can be determined under some lab testing, but most of us don't work in controlled lab conditions - our phones are with us in the real world and that's subject to a lot of variables.
Essentially someone can say Device A is the best signal wise, however that can be different to the next person when it comes to being used in their daily lives.
So yeah.... Not sure your going to find any definitive answer, but only by maybe asking people in the areas your finding weak signal what they are experiencing and what network they are on.
Well, I just ran a dB test on my Note 4 and 5C with putting in my SIM card into each phone and I found that the 5C had -120 and the Note 4 had -104. Note is much better, but I don't have a 6+ or Nexus 6 to try. I am thinking about just going to my carrier and testing their demo units to compare them to mine at one or two stores to get a more definitive relative sample size.
My experience has shown me Samsung phones are the best ones with signal strength, at least the flagship ones.
Which phones would you be referring to in addition to your S5?
Previously I had the S3, also excellent in signal strength, and my family has two Samsung midrange phones that are good also, but maybe not as good as mine.
Okay, I requested a replacement Note 4, but if things don't get better, I might just sell it to someone on Swappa or eBay as a mint device since my network issues are not due to the phone. My replacement ought to be a brand new unlocked device if anything as well. I could even sell as brand new if I wanted to, but I shall see.
Fingers crossed for you . Keep us posted when you get your replacement.