What is the best router for my parents house? Must come from Best Buy.

The modem is in an upstairs corner of the house. They use it primarily for streaming movies (Fire Stick), basic laptop use, and basic cell phone. It's usually 1-3 devices at once, but the most intense task is movie streaming. The TV is approximately 40-50 feet away, laptop is about 40-50 feet as well.

I need a dual band for streaming, correct? Can someone point me to the best router at Best Buy for the tasks we are trying to accomplish? We don't need any fancy unnecessary features.

You don't need dual band for streaming... really, 2.4GHz and 5GHz are two different beasts. 

In general, 2.4GHz has better range while 5GHz is better resistance to interference (from other wifi routers). 2.4GHz is older and is supported by pretty much all wifi enabled devices whereas 5GHz is unsupported by many devices (Roku 1/2s and Chromecasts, do not support 5GHz... PS3, Xbox 360, Wii U, PS4 also don't support 5GHz). 5GHz is faster than 2.4GHz but for streaming/gaming/most things, you won't see any difference-- only when you download something will you benefit from 5GHz since it'll download faster. 

I need 5GHz b/c I live in an apt and there's way too much 2.4GHz interference. With that said, many of devices do not support 5GHz so for those, I have to use 2.4GHz. I've used tons of them but ultimately settled on the Asus RT-A66U, which works well enough for my budget (I got a refurb unit for $60- its like $100 new). 

Pretty much any decent router these days have dual bands though. The TP-Link AC1900 is generally considered the best bang for your buck, if you can afford it. There are much more expensive/powerful routers but those are probably overkill. Yes, it has tons of features you won't need but in terms of boardcast range and consistency, it's heralded as one of the best. 

You didn't give a budget, but the best ~$200 routers are the Asus rt-ac86p or the Netgear Nighthawk AC1900 (R7000).  Both are available at Best Buy.

Both are future proof (support G, N, and AC bands) and are very highly rated.  My experience with really cheap routers (sub $100) is that they are just pure garbage.  Do you want to wait for Netflix to buffer because of a janky router?  Range in a house can also be an issue, so the multiple antennas on either of these routers will help.

Asus Ac68 is also a good router with good range. Expensive at around $150 or so. But your budget is a big thing. Most good routers are over a $100.

And you mention the distance from the router. Is there a lot of walls between them. Is the current one not able to keep up or has bad signal/range?