EF ICollection Vs List Vs IEnumerable Vs IQueryable

IQueryable:

• Query isn’t executed until you really iterate over the items, maybe by doing a .ToList() or a foreach. Which means you still can add filters, like a Where().
• Extends IEnumerable

IEnumerable:

• Forward-only list of items. You can’t get at “item 4” without passing items 0-3.
• Read-only list, you can’t add to it or remove from it.
• Still might use deferred execution (IQueryable is still an IEnumerable).

IList:

• Random access to the full list
• Probably entirely in memory (no deferred execution, but who knows what the exact class does that implements this?)
• Supports adding and removing
• Extends IEnumerable and ICollection

ICollection:

• Is between IEnumerable and IList.
• Extends IEnumerable

What is “best” depends on your requirements. Usually though an IEnumerable is “good enough” if you only want to display items. At least always use the generic variant.