I personally use [target=targettarget]

Which means your offensive spells hit your targets target

Here's the one I use:

/cast [@target,harm][@focus,harm][@targettarget,harm][@focustarget,harm] Smite

What it does? Well, it uses the "harm" condition to reject any candidate for Smiting that isn't an enemy. First it tries your target, then your focus, then your target's target, then your focus's target. So you could either focus the tank or the boss, depending on the fight (or which is more consistent). If it's important that your smites hit the boss every time, then focus him/her.

Word of advice to macro writers: "@something" is the same as "target=something". Saves you a lot af characters on longer fallback logic!

You don't need the 'harm' directive for a spell like Smite that can only harm, it's implied. Smite won't cast at a friendly target, whether you've got harm in there or not.

Penance and Holy Shock are spells you might need to use the 'harm' and/or 'help' directives for.

You can also save a character with 'use' now, instead of 'cast':

/use [@focustarget] smite

or more complex:

/use [mod:ctrl] ability4;[mod:alt] ability3;[mod:shift] ability2;[nomod,@focustarget] smite

I set my assigned tank to my focus, so using that macro Smite will always hit my tank's target. If you've got Attonement, this ensures that your assigned tank is always in Attonement's 15yd healing range.