I work in a very weak signal area, so I am always searching for the best Android on Sprint for RF signal strength. I've had a lot of phones and experience in regards to weak signal area testing. I've tried the original Motorola Photon, Motorola Admiral, HTC Evo 4G (original), HTC Evo 4G LTE, Samsung Galaxy S2, S3, and S4, and the LG Optimus G, among others. I always have roaming turned off, so this is strictly using Sprint...
The way I would rank the best ones that I have tested here at work for RF signal reception:
1. Motorola Photon (original version, not the Q) - Hands down the best, so far...but too damn old!
2. LG Optimus G (performs much better than any other when on weak 3G & 1x, this is my phone of choice right now. Very consistent.)
3. Samsung Galaxy Victory (On par with the Optimus G for average strength, though slightly less stable; weak 3G performance is poor in comparison, however.)
4. Samsung Galaxy S3 (Decent, but inconsistent; weak 3G is pretty unusable.)
Anyways, that's what I've come up with so far! I'm gonna try a couple of mid-range LG's (The Viper & the Optimus F3) and see how they perform, since I like the RF performance of the Optimus G so much; maybe these two would have similar or even stronger strength. We'll see! :-)
Cheers,
VegasRooted
PS-- As others have mentioned, I have also heard that the Samsung Galaxy Note II is extremely good for RF signal, though I've not tried it for myself. It is just a little too big for my taste...Also, the Galaxy S4 actually performed weaker than the S3 for me in the same weak signal locations...too bad, I love the hardware. :-(