[Galaxy S 4 Q&A, Help & Troubleshooting] tuk1maar:
I made an decision myself a few months ago. I chose the GS6 and returned it and kept my Galaxy S4. I chose to wait out for the Galaxy S7 and other flagships to be released. But, if you really want a superb phone right now, read below!

First in short:

1. Galaxy S6-series/Note5: Most reliable Android phone on all grounds. Most reliable camera for a 2015 smartphone and has a long-lasting battery. Weekly updates from Samsung which improves the phone are also there! No SD-card expansion here!

2. Moto X Play: Reliable AFFORDABLE!! Android phone that does not crash just like an Galaxy S6. It has a much lesser camera, yet still better than our Galaxy S4. It offers an SD-card expansion slot, AMOLED-LIKE display with brightness rivaling the mighty Galaxy S6-series. Battery-life is this device' top point. It easily outclasses every other modern flagship smartphone.

3. LG G4: Reliable Android phone. Not a reliable camera as the Galaxy S6 (bad dynamic range, bad OIS in certain situations, slow auto exposure-adjusting and many more) BUT when it does work, it produces amazing results in specific situations. Offers mediocre battery-life. Expect yourself to become a wall-hugger, although not as much a wallhugger as people with an iPhone 6(s). It DOES offer SD-card expansion AND a user-replaceable battery!

In details:

Personally, for a reliable phone, I'd pick the Galaxy S6-series flagship model. It's simply the best all-rounder with strong points at every aspect. The Galaxy S6 is imho very cheap for an flagship and is just plain awesome.

If you are on a budget, I recommend the Motorola Moto Play. It's truly a flagship killer, except for it's camera. It's camera is vastly better than what we have on our Galaxy S4, but it does not get close to what an S6 can produce. That said, the camera is very usable! The Moto Play has a truly astounding 5.5" IPS display, about as ridicilously bright as an Galaxy S6 can get and has a ridicilously high contrast ratio (very AMOLED-like!!!). It's killer point isn't it's display though, neither it's speed (keeps up with an Galaxy S6). It has INSANE battery-life! It smokes all Android flagships and Apple's iPhone 6S aswell when it gets down to battery life! Truly, the Moto Play is my top choice for EVERYONE that is on a budget, or is looking for an insanely fast and RELIABLE phone that has mindboggling battery-life. On a side-note, the Moto Play also offers MICRO SD-CARD EXPANSION.

The LG G4 isn't a great choice anymore. It has a lackluster display due to a very low ,maximum brightness, feels sturdy yet cheap. Camera is amazing in specific situations while in other specific situations, it performs worse than even our Galaxy S4 (microphone when outdoors, video in 1080p, the LG G4 is terribly oversharpened while our S4 makes it look smooth like you'd expect). BUT, it does offer user-replaceable battery and a micro sd-card slot! In my opinion, I never swap batteries and only use an SD-card slot.

For both the latest Galaxy S series flagship and the Moto Play you do not need custom roms for stability and reliablity. For the Moto Play you might want custom roms for extra features that the Galaxy already offers out of the box. The LG G4 is a mixed bag here. It's a playfull device, not as feature-proof as the Galaxy S6 is but there ARE custom roms availible for it!

I could give you way more input than this since I research smartphones for my own business regarding what device suits specific people the best.