If I could only tie one WB, I would tie a golden olive on either sz 6 or 8 with a gold cone head. Don't be afraid to go bigger to a 6. The fish will hit it, and you will be able to cast it no problem. I don't like 'light olive' (too 'lime coloured') or dark olive (too turquoise) as much, but these are also good fish catchers. If 'golden olive' is not listed, and you have a choice of just 'olive' then that's usually the right one.
Make sure you get longshank hooks. I like the daiichi 1750's. And some gold cone head beads. 4.5-5mm will fit 6-8 LS hooks. You shouldn't need a lead underbelly with a cone head on a 6-8. I like fine chenille for the body, but that's just me. Some people use cactus fritz or disco fritz or really bushy chenille. I use 3 strands 'midge' crystal flash each side in the tail (midge flash is more flexible). For size 6-8 I like 2 marabou plumes in the tail. (I think a bushy tail is key to this fly).
Its worth spending some extra money on decent marabou - it will hold up longer and be more uniform when you tie it in - hareline or wapsi perhaps. Same goes for the hackle feathers. You need surprisingly long feathers to properly palmer hackle a WB. At least 7 inches for a size 6. If you buy the cheap stuff, you will get a lot of worthless webby feathers. I've had good luck with the Hareline bog standard strung chinese saddle hackles in olive (5-7 inches). I've had BAD experience with their 'grizzly variant' strung saddle hackle, which was very webby. I rib my WB's with a bit of wire to keep the hackle in place - you'll need it when you start getting into those cannibalistic browns with big enough teeth to cut your thumbs and fingertips up! That's a lot of stuff to tie in at the tail of the fly, so you might need to build up the rest of the body a bit before you wrap the chenille. I find the leftover marabou works great for this - just dub it on and wrap forward like dubbing fur.
If you want to have the best shot at catching the biggest fish, then you do indeed need to get a sinking line and fish on the bottom, but sometimes even the biggest fish will come up a bit if the fly is fished right. Guys who fish WB's on floating lines do really well too. And its a heck of lot more fun than casting a DT line! The trick is to give it time to sink - cast upstream and throw a big upstream mend and then let it tumble down to your target area on a slack line before you let it swing. Tighten up the line and let it swing when it gets where you think the fish are. In this way, you trigger that strike response of a fleeing baitfish, which I think can be more effective than imitating a fish muddling around on the bottom.
And of course, if you are feeling lazy, you can try just casting straight downstream and retrieving with a stop and start figure of 8. This works surprisingly well, even if it does get a bit boring.
Good luck.