It depends really on your ordered options (as usual). For those 737s that are CAT IIIb capable is autoland with flaps 15 possible since that includes CAT IIIa single engine capability.
Apart from that we had to use flaps 30 or 40 in an autoland on the classics and flaps 40 only in real CAT III approaches. On the NG it was changed to either flaps 30 or 40 in either circumstances except for single engine operation.