So, in order to answer your question completely, we need to know what the computer is for. Is it for gaming? Watching movies? Digital art? Reading/editing documents? Homework? Office work? Browsing the web?
A lot of those will have a different answer from the rest. There is no "best computer". No computer is best for everything. If it's for art or video editing, you'll want one with a decent amount of RAM and a fast, dual-core processor(around 3-4GHz at least) with a large, VERY high-res screen.
If you're gaming, you'll want a very similar computer to the one just described, but with even more RAM(at the very least 8GB) and a processor with more cores(4-6). The more cores a processor has, the more multitasking it can do. That's because each core is actually another whole processor built to work in tandem with the other cores, each reading its own line of code. And running high-res games requires a LOT of multi-tasking. That's because there are models to render, textures to apply, physics to calculate, controls to analyze, etc., etc..
For both of the above, you'll probably want a HDD around 750GB at least, most likely closer to the 1-2TB range.
If it's simply for browsing the web, you can get one that's MUCH cheaper that will work just as well. Why spend $1500 minimum on a gaming computer to just use it for Facebook? You'd never use all that processing power you just paid for. 2-4GB of RAM, 300-400GB HDD, a regular screen size(11-14'' is fine), and a dual-core processor around 2GHz.
A computer for office work can have the same specs as a web-browsing computer, but you want to make sure it's a good brand that will last with minimal problems. So, if it's a work computer, you'll have to fork out a little extra. But nowhere near as much as a computer used for gaming or 3D modeling.
I hope one of the above build recommendations fits what you're looking for. If not, just update the question or send me a private message and I'll do my best to help out. If you couldn't tell by now, computers are kind of my thing. :P
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