Going back a few years, I used a 6100 as my main Mac at home. In the end I opted for Mac OS 8.6, because it offered the best combination of performance (more of the Toolbox is PPC native than 8.1 and earlier), unbugginess (8.5.x was pretty unrelable, in my experience) and compatibility with most mid 1990s applications. You may even be able to run a web browser that works with modern sites.