You didn't say where it's leaking from? If the rear main is leaking, you're probably going to have to pull the crank...
On my most recent build I used the BOP 1-piece oil pan gasket. I like it a lot. It's a steel core gasket that's encased in rubber with ribs on both sealing surfaces. The steel core keeps it from 'squishing out' when you tighten all the bolts.
For valve covers, I used the Edelbrock 7590's. For intake manifold, I used the Felpro's that have the raised silicone sealing bead around the ports. For headers, I really like Remflex. For the rear main, I used one of the new BOP 1-piece seals. I had trouble this time getting mine to seal, mostly because of the (mis)shape of the aftermarket oil pan I'm running. To make sure, I put the engine together, filled it with 2 gallons of oil, and hung it from my hoist "butt down" to make sure the entire rear of the crank was submerged in oil for two days to see if it was going to drip. It took me a few tries because of the pan, but I did finally get it sealed up.
I used the good GM silicone gasket sealer - the kind that comes in a tube and needs a small 'caulking gun' to apply - on just one surface of the few gaskets where I used any sealer at all. One of the tricks is to apply a small bead of the stuff (don't go crazy, you don't want globs of it squishing out where it can circulated around in your oiling system), install the parts with the bolts LOOSE (just tight enough so that the sealant is touching both surfaces), let it cure for about an hour to start firming up, then tighten the bolts. If you don't have an inch-pound torque wrench, get one. It's not hard to overtighten the pan bolts and valve cover bolts enough to dimple them and cause leaks. I also put small dabs of it in the "corners", like where the timing cover, block, and oil pan meet and also where the heads, block, and lifter cover meet.