Expected Transmission Time (ETX) is a best parameters to calculate the link quality.

RSSI is the another parameter to calculate the link quality.

RSSI is related to the transmission rate .

Expected Transmission Time is one of the parameter.

Received Signal Strength Indicator is another parameter.

Channel State Information is another parameter.

The RSSI (Received signal strength Indicator) measured by the 802.15.4 devices enable the algorithm to get the Link Quality Indication (LQI) of the channel. These metric was used to calculate link quality in the network. The ZigBee standard builds on top of the IEEE 802.15.4 MAC and PHY layers and encompasses a complete network stack for WSNs focused on sensor and control networking .

ETX is an active monitoring type metric and also used a bidirectional transmission. This is mainly used in receiver side. From this parameter, the number of packet transmissions was calculated. It applies the successful forward delivery ratio and successful reverse delivery ratio to select the path with the highest packet delivery ratio. It is mainly used to reduce the number of transmissions for data packets.

This is a good reference:

http://dl.acm.org/citation.cfm?id=2240123

The choice of a strategy to estimate channel quality is application-specific. For example, for very reflective environments, the use of raw values of RSSI can provide wrong conclusions, due to the random variation of the RSSI caused by the multipath fading. In scenarios with burst interference the LQI can overestimate the channel quality, since the LQI of lost packets are not accounted for. Metrics based on network-layer information (also called software-based estimators) sometimes requires extra processing from the sensor nodes, and the transmission of diagnosis packets (active estimators), which can be prohibitive for some applications.

I hope the paper be useful for you.