Usually my methods are as the following:
public List Method1(int input)
{
var output = new List();
//add some items to output
I almost always go with List because it gives me methods that I frequently find the most useful.
One potential negative consequence of returning an IEnumerable is that any exceptions that get thrown upon enumeration would come from an area of code that could be quite far away from the code that actually built the object, making bug tracking harder.