Big Reactors

Basically fuel rods generate 3 things: heat, fast neutrons and slow neutrons, both traveling 4 blocks in each direction.
Heat needs to be transfered to the reactor by the 4 blocks touching the fuel rod.
Fast neutrons need to be slowed down to be useful
Slow neutrons can either be absorbed by the coolant and converted to heat or can hit another rod and cause it to burn fuel a bit more efficient, while generating more heat.

Each coolant has 4 stats: how well it absorbs neutrons, how good it is at converting neutrons to heat, how well it slows down the neutrons and how well it conducts heat.

 Reactor Moderator/Coolant data
            Abs.    Heat    Mod.    Cond.
Blocks:
Iron        0.50    0.75    1.40    0.60
Gold        0.52    0.80    1.45    2.00
Diamond     0.55    0.85    1.50    3.00
Emerald     0.55    0.85    1.50    2.50
Graphite    0.10    0.50    2.00    2.00
Glass       0.20    0.25    1.10    0.30
Ice         0.33    0.33    1.15    0.10
Snow        0.15    0.33    1.05    0.05
Copper      0.50    0.75    1.40    1.00
Bronze      0.51    0.77    1.41    1.00
Aluminum    0.50    0.78    1.42    0.60
Steel       0.50    0.78    1.14    0.60
Invar       0.50    0.79    1.43    0.60
Silver      0.51    0.79    1.43    1.50
Lead        0.75    0.75    1.75    1.50
Electrum    0.53    0.82    1.47    2.20
F.Electrum  0.54    0.83    1.48    2.40
Shiny       0.57    0.86    1.58    2.50
Enderium    0.60    0.88    1.60    3.00
Fluids:
Water       0.33    0.50    1.33    0.10
Redstone    0.75    0.55    1.60    2.50
Glowstone   0.20    0.60    1.75    1.00
Cryotheum   0.66    0.95    6.00    3.00
Ender       0.90    0.75    2.00    2.00
Pyrotheum   0.66    0.90    1.00    0.60

Abs. - Neutron Absorption - How much radiation is converted to heat
Heat - Heat Efficiency - how efficiently radiation is converted to heat
Mod. - Moderation - How well it slows down the neutrons
Cond.- Conductivity - The higher the number, the more RF you get from geneated heat.

Cryotheum is by far the best at transfering heat and slowing down the neutrons, but it absorbs too much neutrons for many of the reactor designs - it can actually lower the overall efficiency.
Resonant ender has the best absorbtion stat - it will catch 90% of the neutrons that hit it, therefore it's the best for the outer layers of the reactor.

As for rod layouts go, my speciality is active cooled (steam producing) reactors - my two favourites are checkerboard:
http://br.sidoh.org/#reactor-design?length=15&width=15&height=15&activelyCooled=true&controlRodInsertion=87&layout=16E13C2ECXGXGXGXGXGXC2ECGXGXGXGXGXGC2ECXGXGXGXGXGXC2ECGXGXGXGXGXGC2ECXGXGXGXGXGXC2ECGXGXGXGXGXGC2ECXGXGXGXGXGXC2ECGXGXGXGXGXGC2ECXGXGXGXGXGXC2ECGXGXGXGXGXGC2ECXGXGXGXGXGXC2E13C16E&modpack=direwolf20

and  rings:
http://br.sidoh.org/#reactor-design?length=15&width=15&height=15&activelyCooled=true&controlRodInsertion=92&layout=16E13C2EC11XC2ECXGXGXGXGXGXC2EC11XC2ECXGXGXGXGXGXC2EC11XC2ECXGXGXGXGXGXC2EC11XC2ECXGXGXGXGXGXC2EC11XC2ECXGXGXGXGXGXC2EC11XC2E13C16E

The later is (slightly) more efficient, but since it needs to be throttled down more (since no reactor can output more than 50,000mB/t of steam) it's less flexible and can't be turned down for lighter loads.