Honestly there is no "best" computer. But if you only really plan to realistically play CS GO I'd just upgrade your graphics card on your current computer granted that it's recent enough.
If you were buying a new Computer all together then you'll have to think about if you plan to play on just a 60HZ Monitor or a 120-144HZ monitor. But for a good enough gaming PC for CS: GO i'd say a $500 Computer is more than good enough for fit the bill.
You'd be looking at something that has about a GTX 750TI/760/950/960 or an R9 270/270X/280/370/380 for a graphics card. If you plan to build it yourself I can make a rough build so you can see what components you can get on a $500 system