Is best straight hand in Texas hold em poker A-5 or 4-8?

The best "straight" combination in this case is the one 4-8.

4 - 5 - 6 - 7 - 8 is a higher 5 card combination then A (1) - 2 - 3 - 4 - 5.

In this case the A-5 straight (known as "The Wheel") uses the ace as the "number 1".