Knotted leaders allow you the most versatility. And, there is something to be said for playing a fish on a leader you tied. Most people use Blood Knots in the heavy butt and midsections.  They pull straight and true. Tippet sections are often tied on using a Surgeon's Knot. They are stronger and easier to tie than a Blood Knot, but they do pull to the side when tied in heavy mono. 

7 1/2'

 For a streamer leader try this:
24" of .019", 16" of .017", 14" of .015", 
9" of .013", 9" of .011", 18" of .009" 
(.009" is 2X) 		 Short length turns over easily.  Fly stays close to line when using sinking lines. Accurate

9'

 For a general purpose leader try:
36" of .021", 16" of .019",
12" of .017", 6" of .015", 
6" of .013", 6" of .011", 
6" of .009", 20" of .007" 
(.007" is 4X)		 Good general length.  Allows sinking fly to sink deeper, quicker. Allows floating flies a longer dead drift.

12'

 For a longer, leader try:
36" of .021", 24" of .019",
16" of .017", 12" of .015", 
7" of .013", 7" of .011", 
7" of .009", 7" of .008", 
28" of .006" (.006" is 5X)		 Good for longer dead drifts.  Allows sinking fly to sink deeper, quicker.

Pro

 Easy to use.  Tied just right. 

Con

 Heavy butt material tends to coil. Knots catch algae and other debris.  Knots can fail. Knots take time.