Like most comments, it ultimately comes down to your needs as far as what phone is best for you. I'll share my experience with a couple of these devices. Maybe it might help.
I was on the same boat as you recently. I previously had the S3 and also the G2. I upgraded to the G3 because based on all the research I did, I thought is was the best all around phone between all three phones you mentioned. I really liked the Knock On / Knock Code features. Screen looks great. Speakers are pretty good. Camera is great. And it's a great looking design. Even though it's plastic, it looks and feels great.
I replaced it with the Galaxy S5 a couple of days ago. And I am 100% happy with the decision. My reasons for replacing it to the S5 are:
1) The battery on the G3 I purchased was horrible. I had to charge it twice a day after leaving the house with 100% charge. I'm an average user. Don't stream much. I browse web, social media, emails, text and talk...
2) The G3 constantly overheated in the late afternoon each day
3) The G3 would overheat when using navigation or charging it while still in use
4) G3 lagged when running hot which was every day.
5) It was difficult to use with one hand. I have average size hands and even though the physical size isn't much bigger than the S5, the screen is much bigger. Reaching to the opposite end or the top/bottome of screen was a challange with one hand.
6) Althoug I know I could download other keyboards, I was disappointed about a few things on the LG smart keyboard.
A) the word suggestions don't pop up above the keyboard with word options when running Youtube, maps or typing web url's.
B) You have to delete each letter at a time instead of deleting the entire word if it types the incorrect word. Other phones like Galaxy line or HTC's allow you to delete the entire word with one press, which make things a little easier and quicker.
Some of you might figure these settings out but I tried and tried and couldn't. Even with the help from Verizon rep and LG rep.
So, here are the reasons I'm happy to have switched to the S5:
1) Battery lasts much longer than the G3 I had. Example, I left home at 8am this morning with 100% charge, it's 4:30pm and my S5 is at 70% charge. And this is without charging it again.
2) Hasn't heated on me once
3) No lag what so ever. Smooth all around
4) Screen is easier for me to use with one hand. I can reach end to end easily.
5) Feels much easier to use for me. Like, when I'm scrolling through messages or call history, I can swipe left or right to call or text
6) The camera is great as well.
7) It just feels more functionable to me for my everyday use.
😎 The screen is easier to see during daylight
Basically, I haven't found anything I dislike about this phone. Yes, it might have some features I don't need or use but what I do need and use, works great, and does what I need it to. The speakers are probably the one thing I wish were better but I don't use them often. So it makes no difference to me.
Which leads me to why I didn't get the M8. Based on my needs, I felt the only things the M8 might be superior at are the speakers, design and maybe the UI, HTC Sense (if you're into that UI). But for me, the speakers are not that important to me. I don't use my phone speakers much. Not for music or calling. The design, I don't find too important what material the phone is made out of. I always end up buying a case anyway so what's the difference (to me at least). I mean, I'd rather protect my phone investment from drops and/or potential damage so the material is irrelevant to me.
Anyway, sorry for the long comment but thought I'd just share my experience since I was in a very similar boat as you. Good luck!
