Are there any other preferences that you want to set yourself?
- Screen size/dimensions size? A phablet?
- Design?
- SD card or removable battery?
- UI of the stock OS skin?
- Raw power?
At the moment, the flagship phones of this year are the
- S5
- HTC One (M8)
- Xperia Z2
- LG G2
- and the new cheap alternative, the OnePlus One.
Other Android phones that are due an update in one month are the Galaxy Note 4, Galaxy Alpha, Moto X +2, Xperia Z3 or phablet successor, the LG G3 Prime and the HTC One Max 2. These are mostly phablets except the Moto X+2 and maybe the Alpha.
The other choice would be the Nexus 5 , which is getting a refresh November/December time, but that may be a bit way off if you want to wait.
First, decide what you want out the phone, because it's hard to advise on the parameters you gave, since there a number of phones.
Don't forget, if you want to go smaller, you can get the min versions of the HTC One Mini 2, G2 Mini, and S5 Mini, Sony M2 (around 4.5 inches), but these are mid range phones instead of flagships.
HTC One M8
- Premium Design aluminium unibody (nice but imo not as good as the M7)
- SD card slot
- Best UI (simple and not as ugly as TouchWiz) and Blinkfeed (preferential) - if you are not rooting
- Best Selfie camera (if that's the thing for you ) 5MP front
- Poor camera in low light conditions
- Speakers much better thany any other phone (due to deual setup)
- Battery the worst out of flagships (2600mAh)
- More accurate instead of brighter colours
- Samsung Galaxy S5
- Waterproof, dust resistance
- More durable
- 2800mAh battery
- removable sD and battery
- Vibrant good screen, very bright Super AMOLED, 5.1 inch
- Touchwiz (pro or con to you)
- Design worse (preferential)
- S Health features
- physical buttons
- plastic questionable design (preference), better for grip
- Viewing Angles better than G3, very wide
- Multitasking
- Sensors for eye tracking, etc.
LG G3
- 538 ppi QHD IPS Screen (worse for battery but can reduce to Full HD)
- UI questionable, heavy like Touchwiz
- On screen buttons
- Design plastic but masked (metallic)
- Back volume button rocker (preference)
- similar to Samusng, might feel at home coming from S5
- Multitasking
- Sensors for eye tracking, etc.
- 3GB Ram Snapdragon 801 (most powerful)
-Much more like a phablet (5.5 inches)
Xperia Z2
- Premium Design (Glass and Metal) - Harder to grip
- Dust/waterproofing
- 5.2 inch screen, IPS, more natural, not as bright
- UI closest to stock Android, cleanest and most simple
- 3200mAh battery but battery not as good as others
OnePlusOne
- VERY Cheap
- 5.5 inch phablet screen
- great battery (due to being bigger)
- Lack of micro SD card slot
- Cyanogenmod
- 3GB RAM
-
HTC One M* has a great case too (pixel case)