A fun question for Biology/Chemistry A-Level students etc.Watch
Mine is definitely the keto acids (molecules that have both a carboxylic acid and a ketone functional group). A good example of that is pyruvic acid. It's involved in many biochemical processes, involving the Krebs / citric acid and glycolysis.
I also love esters (what you get when you react carboxylic acids with alcohols under acid catalysis and heat) because they give off such amazing fragrances (methyl salicylate smells like wintergreen, pentyl butyrate smells like apricot, amyl acetate smells like banana etc.) But not just because of that. They're also amazing plasticisers (additives which lower the viscosity of other substances) and they're completely safe to consume too, unlike most organic molecules. They are also really good solvents, for lacquer etc.
If hybrid homologous series like this don't count, then I'd probably go with amides, because it's amazing how from something like amines (which smell like rotten meat or fish) you can get such a useful family of molecules which are prevalent in our DNA, in drugs, in plastics, proteins etc.
Arenes are also cool but it's not really a proper homologous group since so many complicated molecules have a benzene ring in them and their properties are so different.
I like the esters too for the same reasons you outlined haha.
I might go rogue and say the azanes though: they are a homologous series of saturated hydronitriles, i.e. straight chains of nitrogens and hydrogens with single bonds, like H2N-NH-etc...-NH2. General formula NnHn+2. They're cool because the include ammonia, a very important molecule in biological systems and in industrial processes like fertiliser manufacture, and hydrazine (H2N-NH2), which has many uses. It's used as rocket fuel, as a propellant onboard space vehicles like the NASA Dawn probe and used as an ingredient for making lots of pesticides and pharmaceuticals (especially for forming rings containing N-N, like pyrazoles, and nitrogen-containing heterocycles where you have multiple rings fused together). It also has a really interesting 3D shape, which you can predict using molecular orbital theory. The two NH2 units are twisted relative to each other! If you study chemistry at Uni you'll end up encountering hydrazine relatively frequently!! (I study physics now but in my first year I had some chemistry too).