Best armour is leather everything plus a steel helmet, sword, and shield. The absence of heavy armours give your dwarves greater mobilitiy, and ease in dodging. Well-trained swordsdwarves can butcher targets very, very quickly. I came to this conclusion after running some arena tests 10v10.
Lightly armoured troops are much better at intercepting thieves too.
But don't take my word for it...
Run some tests. The best way to discern what is useful in a given circumstance is to run arena tests against the sort of opponent you expect to be up against. I found fencers worked best against moderately-armoured goblins at similar skill levels.
These interceptors are your kill team.
In a more complex siege, you may have multiple entry points. It can help to have a few heavily armoured dwarves stand guard at an entrance. With a high enough armour skill, they will be more durable against invaders, effectively slowing them down... while your interceptors clean up elsewhere. Just be sure that your 'tanks' engage first - so that the goblins concentrate on them, rather than on your leathers.
Might be good to have a couple of hunters out and about searching for ambushes...
And then, if you have enough lumber, you might want to load out all of your civilians with cheap crossbows. For effect.