As *Cane stated the rifle that you have the most rounds for. My choice without reservation would be the AR-15 Rifle with an ample supply of 5.56 or 223 Rounds. Preloaded in magazines!
There are a lot of reasons for my determination.
1. A good AR-15 Rifle is very reliable
2. The AR Rifle is very accurate
3. It is easy to clean and keep running
4. There are literaly thousands of them out there accounting for those obtainable from Military,Law Enforcement and the civilian population.
5. Parts are readily available
6. Large quantities of ammunition can be stored in 30 round magazines
7. Reloading is the quickest of most all rifles
8. Volume of fire is as good as it gets for semi-auto rifles
9. Wear on the AR is minimal compared to amount of rounds fired
10. An AR pointed at you with a 30 Round Magazine in it is pretty intimidation and should be!
Just some reasons why I would prefer a good AR Rifle or two with me if the SHTF. Also I would have a good pistol 45 ACP or at least 9MM along for the ride.