I decided to start trying to make redstone contraptions, because it's a big part of the game and i hardly use it at all, so i was like "haha, I'll generate a world for Redstone, so I can test it out"
I then used the seed: "Redstone" (without the " and with the capital R)
Heres why its good for clay:
You spawn on 1 or 2 stacks of it.
If you travel (by boat preferably) straight out into the ocean from your spawn, and peek about the islands roughly a km away you'll find a lot more. I have roughly 9 stacks of it just from that.
This seed is by far the best one i've got yet, and for clay, you NEED it!
Remember, you spawn RIGHT ON TOP of like 2 stacks!
Only problem is, if two people use the same map seed, they can end up with spawn points in completely different areas of world, despite both people having been spawned in an identical map.
Also, I'm glad I mostly play on an smp server. We can pour water over gravel to turn it into clay. It means bricks are much more plentiful (because in real life bricks are a very common building material), and it actually makes gravel useful.