Your question is too broad. It really depends on what you do on your old Mac.
For me, I sometimes browse the web. In this case, hands down, Netscape 2.0 is the way to go. There is no graphic browser any faster than this.
For word processing, I use Word 5.1. But WriteNow is an excellent alternative.
For maintenance, I use Central Point Software's MacTools Pro 4 and also Norton Utilities 3.5, in combination with Apple's Disk First Aid. I also use Conflict Catcher to handle my CP's and Extensions.
For fun, try The Manhole or Cosmic Osmo. Crystal Quest is also a great time killer.
For best system performance, use some flavor of System 6. For more features and newer software compatibility, System 7.1 is the fastest version of System 7, but 7.5.5 has some bells and whistles that you may enjoy.
For great B&W graphics creation, I recommend Canvas 3.5.6.