2 dimensional array VS excel as database

First question is, what do you mean with efficiency, speed of retrieving data, looping data, amount of used memory, coding speed and reliability...?

If you mean access speed in your program, an array would most likely be the fastest from these, but where does the data come from? Also if you need data persistence, how do you store the data. If you use custom in-memory structures you most likely have to build a lot of supporting operations (read, save etc) to fulfill the requirements.

Excel doesn't qualify as a database and I wouldn't use it as a data store. Also as Simon stated the interop can be a mess.

SQL Server is a database, it offers data persistence, transactionality etc, but retrieving and modifying data is typically slower than with in-memory data structures. However, it offers a wide variety of operations for data handling out-of-the-box so coding would be easier compared to custom solutions.

So without more background information it's hard to put these in order.

One option you didn't mention is a datatable. As you can use a datatable for storing the data and it also has WriteXml and ReadXml methods it could be one possibility if the data amounts are small and reading and writing to disk are seldom operations.