On lower spec'd portables, I like Tiger 10.4.11 and for the 1ghz and up machines, I like unofficial Sorbet Leopard 10.5.9. There are linux options like Macbuntu and Lubuntu remix which are based off 16.04 iirc, but my understanding is that the software repos were turned off, so software is limited out of the box. Additionally, for powerbooks, I had alot of trouble with video drivers. It is worth noting that I am in no way a "good" at using linux. I like it and use it daily on my early Intel macs, but my linux wheelhouse is pretty basic, so Im sure there are those on here who could help you with that like
@wicknix for example where I could not. Everything I speak to below this is for Tiger, and/or Leopard and Sorbet.
Browser: DarkWeb and InterwebPPC. DW is a bit lighter but both are great for surfing the modern web.
productivity: Office 2004 and iwork
YT: this requires at least leopard 10.5.8 but Tenfivetube is solid. I also like to use an alternative front end like invidious in an IWPPC Box leveraging something like "In the box".
webmail: I use boxed up iterations of outlook.com and html gmail via Interweb PPC but others have had good success using the mail client in Sorbet Leopard.
online shop - I use boxed up iterations of amazon & ebay.com to run search/research. For actual transactions, I use my iphone14 lol.
Games: Quake3A, UT99 etc. Tinkerdifferent hosts some servers, so if you want to get your FPS on, this is an easy lift and so much fun.
There's other apps I use on my PPC machines but that is the primary DD app lineup. There is of course a bunch more - lots of neat apps folks have put together that Im sure you'll discover with time. Alongside this forum, there is a vibrant PowerPC community at forums.macrumors.com. Lots of knowledgable folks over there as well, so if you haven't, go lurk. Many of us exist in both forum spaces. Anyways, most all of this stuff you can find at
the garden - conversely, if you went with sorbet leopard 10.5.9 for example, that community os update has a really convenient PPC application download manager which for those new to PPC macs, is very handy making hunting down PPC appropriate software easier out of the gate.
Best of luck to you.