I have yet to see anything that outperforms this one when it comes to a combined judging of power output, efficiency etc.:
code: 0D140D0000000000000C0D0C0000000000000D140D0000000000000D0C0D000000000000010101000000000000010101000000000000
Since it is a single chamber reactor, safe and easy to automate, you can simply pack a great number of them together. I liked to automate whole banks of them with the Router. If you don't have as similar option then you can do it very simply with some EIO conduits(if you use the colour channels you don't shouldn't need to filter every connection on the reactors, just the output destination of empty fuel cells)(great for running redstone conduits in the same space) or similar as well.
You also save materials/processing steps on not having to make upgraded fuel cells, since this one makes proper use of breeding.