Forced convective heat transfer is experimentally investigated using aqueous and ethylene glycol-based spherical titania nanofluids, and aqueous-based titanate nanotubes, carbon nanotubes and nano-diamond nanofluids. These nanofluids are formulated from dry nanoparticles and pure base liquids to eliminate complications due to unknown solution chemistry. All the formulated nanofluids show a higher effective thermal conductivity than that predicted by the conventional theories. Except for the ethylene glycol-based titania nanofluids, all other nanofluids are found to be non-Newtonian. For aqueous-based titania and carbon and titanate nanotube nanofluids, the convective heat transfer coefficient enhancement exceeds, by a large margin, the extent of the thermal conduction enhancement. However, deterioration of the convective heat transfer is observed for ethylene glycol-based titania nanofluids at low Reynolds numbers and aqueous-based nano-diamond nanofluids. Possible mechanisms for the observed controversy are discussed from both microscopic and macroscopic viewpoints. The competing effects of particle migration on the thermal boundary layer thickness and that on the effective thermal conductivity are suggested to be responsible for the experimental observations.