Here you have a nice list of 11 IoT protocols you need to know about.
Here is a summary in case the link someday breaks
Bluetooth Standard: Bluetooth 4.2 core specification Frequency: 2.4GHz (ISM) Range: 50-150m (Smart/BLE) Data Rates: 1Mbps (Smart/BLE)
Zigbee Standard: ZigBee 3.0 based on IEEE802.15.4 Frequency: 2.4GHz Range: 10-100m Data Rates: 250kbps
Z-Wave Standard: Z-Wave Alliance ZAD12837 / ITU-T G.9959 Frequency: 900MHz (ISM) Range: 30m Data Rates: 9.6/40/100kbit/s
6LowPAN Standard: RFC6282 Frequency: (adapted and used over a variety of other networking media including Bluetooth Smart (2.4GHz)
or ZigBee or low-power RF (sub-1GHz) Range: N/A Data Rates: N/A
Thread Standard: Thread, based on IEEE802.15.4 and 6LowPAN Frequency: 2.4GHz (ISM) Range: N/A Data Rates: N/A
WiFi Standard: Based on 802.11n (most common usage in homes today) Frequencies: 2.4GHz and 5GHz bands Range: Approximately 50m Data
Rates: 600 Mbps maximum, but 150-200Mbps is more typical, depending on
channel frequency used and number of antennas (latest 802.11-ac
standard should offer 500Mbps to 1Gbps)
Cellular Standard: GSM/GPRS/EDGE (2G), UMTS/HSPA (3G), LTE (4G) Frequencies: 900/1800/1900/2100MHz Range: 35km max for GSM; 200km max
for HSPA Data Rates (typical download): 35-170kps (GPRS), 120-384kbps
(EDGE), 384Kbps-2Mbps (UMTS), 600kbps-10Mbps (HSPA), 3-10Mbps (LTE)
NFC Standard: ISO/IEC 18000-3 Frequency: 13.56MHz (ISM) Range: 10cm Data Rates: 100–420kbps
Sigfox Standard: Sigfox Frequency: 900MHz Range: 30-50km (rural environments), 3-10km (urban environments) Data Rates: 10-1000bps
Neul Standard: Neul Frequency: 900MHz (ISM), 458MHz (UK), 470-790MHz (White Space) Range: 10km Data Rates: Few bps up to 100kbps
LoRaWAN Standard: LoRaWAN Frequency: Various Range: 2-5km (urban environment), 15km (suburban environment) Data Rates: 0.3-50 kbps.
Just consider that:
The longer the distance you want to cover with the signal the more power consumption you need.
The higher the data rate you require, the higher the frequency thus more power consumption.
So I suggest to go for a low frequency protocol; ZigBee works quite well, consumes very low and it is quite popular. The only drawback is that Raspberry Pi does not include a ZigBee transmitter, you may require an extra adfruit.