Electrically conductive polymer composites (CPCs) based on insulating polymer matrices filled with carbon nanotubes (CNTs) can also be used as potential thermoelectric (TE) materials for converting waste heat into electrical energy. The advantages of polymer-based TE materials over typically used metal oxides such as bismuth tellurides are not only their better availability and cost efficiency, but also their ease of processing, flexibility, low density and low thermal conductivity. Based on previous work on PC with MWCNTs and PP with SWCNTs, it was the aim of this study to reveal the influences of the CNT and polymer type and CNT content on electrical conductivity, Seebeck coefficient (S) and the resulting power factor. Different commercially available CNTs were used to produce melt-mixed composites from different thermoplastic polymers on a small scale. CNTs typically lead to p-type composites with positive S-values, meaning that the charge carriers are holes. The different CNT materials result in different dispersion states, which strongly influences the electrical conductivity. When comparing two MWCNT materials at a constant content, the composite materials with higher electrical conductivity also show a higher Seebeck coefficient, which leads to higher power factors. The selected SWCNT material results in the highest S values among all CNTs at 2 wt.% content and reaches values up to 60 microV/K (in PBT). The type of polymer matrix also influences the electrical conductivity and the Seebeck coefficient. At a constant CNT loading of 2 wt.% MWCNT, PVDF-based composites always showed higher values than those based on PP, PBT and PC. With SWCNTs, the highest S values were achieved in PBT. Interestingly, in polymers such as ABS, MWCNTs behave differently than SWCNTs. SWCNTs are able to produce n-type composites with negative Seebeck coefficients, meaning that the charge carriers are electrons. The magnitude of these negative S values depends on the type and amount of SWCNTs.