The rule of thumb I use is simple.

For speaker wire, buy the fattest / thickest wire you can afford, that is to say the cable with the greatest cross-sectional area ( usually shown in square millimetres ( such as  1.0 mm² or 3.00 mm²  ) Also shown as the number of strands per conductor e.g. 326 Strand

The quality of copper is also important - always make sure you purchase OFC Copper - ( Oxygen Free Copper )

All the speaker cable I sell on Ebay is OFC

Speaker Cable

More copper translates to better overall tone and frequencies being passed to your speakers from your amplifiers

What is also important in speaker cables is how you terminate them.

You want to ensure that you have the tightest connection of the cable to your amp and loudspeaker to minimize contact resistance. For that, I recommend either spades or banana plugs.  

Despite the fact that Spades generally have greater surface area contact than banana plugs, I have found that banana plugs usually make for a better connection to most commercial binding posts found on loudspeakers and amplifiers.

Always remember to keep you speaker cable runs as short as possible and not to loop excessive cables as this may  increase cable inductance.