SSD for Gaming (240 / 250 / 256GB)

For OS/Gaming level loading they all perform close to each other.

Get the "Anandtech Recommended" Transcend SSD370, which is second to none often get overshadowed by Samsung and others, also cheaper. Full review here - http://www.anandtech.com/show/8792/transcend-ssd370-128gb-256gb-512gb-review

With regard to reliability and warranty:

5 Years Warranty:
Samsung 850 EVO

3 Years Warranty:
Transcend SSD370
Crucial BX100
Crucial M500
SanDisk Ultra II

Yes 250GB is ideal for OS/Gaming:

Let's take Samsung 850 EVO - Usually 9-10% goes for SSD Over provisioning (http://searchsolidstatestorage.techtarget.com/definition/over-provisioning-SSD-overprovisioning), which can't be used. So available space is 230GB.

OS: 30GB after all updates
Software: 30GB

3 AAA Games:

GTA V: 65GB
The Witcher 3 - Wild Hunt: 40GB
Batman Arkham Knights: 55GB

Total available space: 230GB
Total space used (OS + Software + 3 AAA Games): 220GB

Update:

I'm surprised you looking for "reliability" and didn't consider Intel 530 240GB in that list.

When it comes to reliability, performance and warranty:

Intel 530 240GB (5 years warranty)
Samung 850 250GB (5 years warranty)
Transcend SSD370 256GB (3 years warranty)

Stay away from Samsung 840 EVO , as their performance is said to decay overtime, which is admitted by Samsung themselves. Everyone knows the recent debacle of inconsistency from Samsung 840 EVO and they almost solved over a period of 7 months after 3 critical updates. But still some experience poor performance as the SSD becomes older and older.
http://blog.neweggbusiness.com/news/samsung-840-evo-firmware/

In the end, I would say get either Samsung 850 or Intel 530. Both reliable, performs well, comes with 5 year warranty. But if you're ok with 3 years warranty, I would suggest get the Transcend SSD370.

Latest Pricing:

PCPartPicker part list / Price breakdown by merchant

Storage: Intel 530 Series 240GB 2.5" Solid State Drive ($119.99 @ Newegg)
Storage: Samsung 850 EVO-Series 250GB 2.5" Solid State Drive ($97.89 @ OutletPC)
Storage: Transcend SSD370 256GB 2.5" Solid State Drive ($89.89 @ OutletPC)

Update on SSD Over Provisioning & Final thoughts:

Yes overprovisioning do play a role in endurance & performance:

"SSD overprovisioning can increase the endurance of a solid state drive by distributing the total number of writes and erases across a larger population of NAND flash blocks and pages over time. It can also improve performance by giving the flash controller additional buffer space for managing program/erase (P/E) cycles and improving the probability that a write operation will have immediate access to a pre-erased block. The extra capacity is not visible to the host as available storage."

Given the better write performance, reliability, the usage of V-NAND in Samsung 850 and above all only costs extra $8 over Transcend, I suggest get the Samsung 850 EVO.

@MarkW - Good you know that you're not part of 5-8% of the people who experienced problems even after all firmware updates. "Samsung has retired the TLC 19 nm NAND memory installed in the 840 EVO in favor of three-dimensional 40 nm TLC V-NAND found in 850 EVO SSDs", that's how serious is the problem. V-NAND is thought to alleviate NAND flash voltage drifting, the culprit—some say—behind read speed performance issues in TLC NAND nodes thinner than 20 nm.. EVO 840 has problems. PERIOD

Cheers!