That is a great question that I get often. There are a couple of things to think about.
In most circumstances rolling your old employer plan into IRA is probably the best option. That option allows you to access low cost funds from places like Vanguard and Fidelity that may not be available in your old employer plan. It also makes it easier for you to keep up with the money, if you change addresses etc.
If you are thinking about rolling your old employer plan to a Traditional IRA and your Adjusted Gross Income (at the bottom of the first page of your tax return) is more than about $112,000 (single) or $178,000 (married) and you do not have a Traditional IRA now, you may want to skip rolling over your old employer plan into a Traditional IRA. Keeping that money out of a Traditional IRA allows you to make a tax-free “back door” Roth IRA contribution by contributing to a Traditional IRA and then immediately converting it to a Roth IRA. In that case you would move your old 401k plan balance directly to your new employer plan if it had lower cost options than your old plan, or leave it at your old plan if the opposite were true.
On the small chance that your old employer 401k balance was in a Roth 401k, I recommend rolling the balance directly into a Roth IRA and not into your new employer plan.
I hope this helps.