For complexity, I was and still am a big fan of Forestry Ethanol. Depending upon how much time I spend in-game and whether my goal is a modest 1000 RF/t or 10,000 RF/t, breeding the best trees for saplings and fruit may takes weeks. Then, setting up the system of squeezers, 1 or 2 fermenters, and a bank of stills is somewhat fun. From there, one can burn Ethanol in RC boilers for steam or directly in TE3 Combustion Dynamos for direct RF/t. I haven't tried piping the steam from RC boilers to a Big Reactor steam turbine, but that might be a fun way to go. I've just usually gone for TE3 Steam Dynamos or RC ISEs.
However, the system to product Ethanol and then convert it to power may not necessarily be the most server-friendly, even if one tries to use the most server-friendly components. There's a lot of leaves breaking, saplings fallings, farms checking for block updates, items moving around, liquids moving around, and plenty of 'machine' blocks doing their thing.
For a much simpler setup that's still fun to tinker/play with and expand upon, Big Reactors seems like a lot of fun. You can start off with a 3x3x3 with less than a stack of Yellorium Ingots and expand from there to 5x5x3 to 7x7x3 and bigger. I haven't verified myself, but others are claiming a decently efficient passively-cooled (RF directly) reactor with an efficient MFR laser with yellow foci is self-sustaining. It gets even more efficient with a Steam Turbine.
Both of the above-described systems are easily capable of 10,000+ RF/t, but the pure Big Reactors one seems more server and client-friendly.
How about Reactor Craft ? Even a fission reactor can produce a lot of power, and a fusion reactor produces an insane amount of power. Does that require a big RoC setup to get up and running ? I really like RoC, but my client can't handle the fancy rending on more than a modest (small) collection of machines/shafts. Tons of opportunity for learning and tinkering there.
Also, any efficient, non-spammy ExtraUtils generator setups ? Automating Whither Boss spawning and slaying for Nether Stars might be fun. That Netherstar Generator is over 40k RF/t.