It will depend on your cell type, downstream application, what you are transfecting, etc. i.e. fibroblasts vs a PBMC subpopulation; short-term transfection for nucleic acid/protein isolation vs longer-term growth; plasmid vs shRNA vs multiple plasmids, etc.

Dear Lindsay, thank you so much for your answer.

I try to transfect primary fibroblasts isolated from the mouse heart.

If you have a suggestion, please let me know....

Thanks in advance, Bodo

Lipofectamine 2000 or 3000 could work in your case. We use the same for plasmid transfection in mesenchymal stem cells. If you want to transfect miRNA, then Polyplus Interferin or Lipofectamine RNAiMAX will work.