I agree with Trev and Doug: By definition, a versatile line can do lots of things.
Likewise, by definition, specialised lines are very good for performing specific tasks (e.g. fishing a big fly at a certain depth).
Since a DT line has the longest usable mass, it can do the most tasks. It's very versatile.
DT lines are:
* Able to cast to the full length of the line*
* Able to cast to short distances
* Able to cast very well in the wind
* Possible to mend at any length
* Possible to Roll cast at any length
* Possible to Spey cast at any length
* Able to be lifted and recast at any length
* Able to present a fly with delicacy or not
* Able to be reversed to extend the life of the line. (Unique to DTs)
* Cheap
I don't know of another style of line that can tick all those boxes, so I'd say DT lines are the most versatile.
Line taper design for versatility is another thing, but generally, simple is better. Get the versatility required by intelligent use of leaders.
Cheers,
Graeme
* They do shoot, but not as well as a WF line at short carry lengths. Get 40' of line in the air and they shoot quite well.