Voltage controller oscillator (VCO) based CMOS temperature sensors are capable of lower power operation (albeit at the cost of sensitivity and resolution) than their BJT counterparts. The power consumption of the sense and conversion blocks is much lower than the VCO itself and hence any further power reduction would be a daunting task. In this work, we compare the performance of three types of VCOs that are suitable for ultra-low power CMOS temperature sensing. The proposed VCOs have been implemented in 150 nm standard CMOS technology, and are biased in sub-threshold region to ensure ultra-low power operation. The frequency of operation for the VCOs is purposely kept low in order to reduce the switching losses. The Differential Cross Coupled (DCC) VCO shows the sensitivity of 1.66Hz/oC with power consumption of 2.5nW. While the Current Starved VCO shows better sensitivity, which is 3.66Hz/oC with power consumption of 12nW. The Transmission Gate-based VCO has the worst sensitivity of 0.28Hz/oC with power consumption of 3nW. Based on extensive simulations, we conclude that the DCC-based VCO is the best choice based on its medium sensitivity and ultra-low power consumption.