Any of the mature web technologies (.net, java, php, etc) can be used to create a scalable REST api. If you application is data intensive, then I would go on to say that more than the web technology, it is the data store that would matter more.
To decide on the data-store, you should first check if the app is read intensive or write intensive. If its read intensive, a RDBMS with a caching solution (EhCache, Memcache, etc) should serve you well.
If its write intensive, you may want some sort of write sharding - you may be better off using a NoSQL solution like Cassandra/ MongoDB, etc.
Also consider licensing costs. With SQL Server most of the scalability features (partitioning, etc) are Enterprise features. With MSSQL 2012, license is per logical core and not per physical processor. If this is an issue, you may seriously want to consider MySQL instead of MSSQL.