thx also i meen by best as in one that i can just setup and leave it alone for a very long time without having to mess with it i dont care about material wise though it could be made out of pure diamond blocks for all i care lol i just want the longest lasting but still good energy rate
look around the this sub forum for Mark I and Mark II.
All Mark I reactors don't build up any heat so, even if you are standing in front of it ready to fill it at the end of every cycle, it will never explode.
All Mark II reactors build up heat during the cycle, but they can complete a full cycle without smelting any pieces inside them, there are designs more stable that can run 10 or 20 more cycle without any cool down (but at some point you will need to wait before using them again) ore there are different ones which can run only few cycles (at least 1) before you will have to wait. the cool down time is usually between few seconds or an hour (there are designs that require more time).
If you never built a reactor follow this instructions:
1) look for a good spot for the reactor and place it;
2) dig a square 3x3 around it on the roof and make the ceiling 5 or 6 blocks high;
3) place a wall obsidian around it*;
4) craft and place six nuclear chambers around it, on for each face of the reactor**;
5) connect a redstone wire to one the chambers from the side and use a switch to turn the wire on or off (you can also connect from the bottom using a redstone torch);
6) connect with the right cable and transformers the reactor to whatever you want;
7) place 9 buckets of water in the 3x3 hole in the ceiling (only one in the middle should also work, but with nine you are sure xD);
8 ) turn the switch and apply redstone power to the reactor, then right cick on it and place your desired pattern;
9) turn the redstone power off, enjoy your fully functional nuclear reactor.
*it's not required.
**some designs don't require all six chambers.
edit: I don't use a lot breeders, but what you should aim for is to make a particular pattern that doesn't gain or loose any heat, then you should raise the heat to 9000+ using buckets of lava and water (lava should be +2000 heat, water -500 if over 4000, ice -200 if over 200). Remember that breeders are not useful to produce energy, they are used to refill depleted isotope cells.
I apologize for my English.