Single-cell RNA sequencing (scRNA-seq) has emerged as a central genome-wide method to characterize cellular identities and processes. Consequently, improving its sensitivity, flexibility and cost-efficiency can advance many research questions. Among the flexible plate-based methods, \u201cSingle-Cell RNA-Barcoding and Sequencing\u201d (SCRB-seq) is one of the most sensitive and efficient ones. Here, we systematically evaluated experimental conditions of this protocol and find that adding polyethylene glycol considerably increases sensitivity by enhancing cDNA synthesis. Furthermore, using Terra polymerase increases efficiency due to a more even cDNA amplification that requires less sequencing of libraries. We combined these and other improvements to a new scRNA-seq library protocol we call \u201cmolecular crowding SCRB-seq\u201d (mcSCRB-seq), which we show to be \u00a0the most sensitive and one of the most efficient and flexible scRNA-seq methods to date.