I want to purchase Macbook mainly for the bioinformatics analysis propose i.e., Transcriptomics, smalRNA, Methylation, lncRNA and other. Would anyone please suggest to me the best affordable one?
It all depends on what analysis you want to run and what organisms you work with. For example, if you want to run genome mapping of RNA-Seq data using STAR for human data takes just a little over 32 GB RAM. This means you wouldn't be able to run it on any of the MacBooks Pro.
If you work with already mapped data, for example, you would need considerably less. Since you also mention "affordable one", I would look for other laptop brands. You can get considerably better laptops from other brands for the same or lower price compared to Apple products.
I think a small server is a better choice for processing bioinformatics data analysis as it is cheaper and more convenient. This is because many analyses can take a long time, and MacBook do not have good heat dissipation.