Many small squads of weapon specialists wins for me. I just feel so satisfied being able to make over 30 station orders of various squads all over the map. 20% marksdwarf military seems decent enough for me, with the rest of the melee dwarves all specializing in their own particular squad, be it mace, hammer, axe, spear, sword or more unusual varieties like the militia's pick (not to be confused with the miner's actual pick), dedicated battle miners who dual wield picks and swords, wielders of exotic weapons like morningstars and daggers (currently confined to were-dwarves) and so forth.
Living sieges are likely to destroy themselves if they arrive on the evil side due to attrition, but in general I prefer having the ability to fall back from one heavy line of defence to the next. Makes things more epic when there is ground to lose - an all important final battle can be prolonged till the last insane Dwarf dies.
As for zombie sieges - divide and conquer.
Gigantic walls are a must for me. A large rampless ditch so that nothing can creep up on the fortifications, and walls below to ensure any possible future diggers are incapable of digging through the soft sand. The loamy ditch itself is encouraged to grow grasses so that should magma ever be easily transported - the entire thing can be rigged to burn.
Everywhere the walls are at least 2 zlvls high to ensure no Titans can climb the things. Fortifications lead into fortifications that lead into interiors that are rigged to explode. Undead can be released at a moment's notice to block out the sun with horrors that go without number. Every room is sequestered into overlapping towers that cannot be fired upon by invaders (although the inverse is not true) and only blockaded against invaders. Fortresses lie within fortresses, towers branch from towers and below it all stands yet another subterranean Fortress that leads into barracks, stockpiles, more undead and the Fort's menagerie. The quarry leads to the surface, trapped to ensure a path is forced for animals to follow a center aligned path - where they can be fired at from marksdwarves stationed behind a ditch + fortifications along the whole thing, with interlocking bridges that form the teeth of crossed keys.
The subterranean Fortress has access to a fully closed water supply with over a hundred and thirty-two wells, access to an above ground and below ground farm which can be cut off form the Fort. If this happens then there is at least two years of food and drink before Dwarves resort to water and starvation. There are nearly 700 weapons for the Dwarves to choose from. There are 100 soldiers out of a population of 149, where 47 are children.
Professional standing military that is a skilled workforce wins all the time. Imagine a Roman military that along with building all this masonry is capable of deforestation, smithing, farming, textiles, crafting and architecture/mechanics.