I remember axes being best for invaders, because they can knock off limbs
Great for megabeasts too. Because they can chop off limbs. Some Titans can only be killed by bisection or decapitation.
while spears are best for large opponents, due to being able to pierce organs better.
Due to being able to pierce armour better and deeper. Works just as well for humanoids as well as Titans. There's also a weird thing in that a masterwork candy spear can cut off Bronze colossi limbs better than a candy axe.
Swords are 50/50 of those two.
Slash, stab and bash. Can't go wrong.
Hammers can't really kill on their own, but a shock squad of heavily armoured hammerdwarves can cripple the shit out of an army for your others to clean up.
Hammers are also great at incapacitating lethally lethal enemies. Like zombies, lashers and *FREAKING WEREBEASTS*. Skull crushing shots are great. You can't really go wrong with hammers 'cept against the big beasts.
Maces are lesser versions of hammers, don't bother.
HE LIES! MACES ARE GREAT!
/notbiased
But seriously, give maces a try. They're good stuff. Less lethal than hammerdwarves, use them against enemy bow/spear/mace/hammer goblnis and animal men/roving wildlife.
Crossbows are pretty broken imo.
Some believe they are underpowered, more still believe they are overpowered. Crossbows are more an objective thing, but are pretty much mandatory for any fort that is regularly invaded by fliers.
*Freaking zombie ravens.
As for me, I keep my squads in squads of 3, each having their own respective weapon/specialty. Spears, maces, axes, hammers, swords, crossbows, elite soldiers, were-dwarves, morningstars, large daggers... All are good. There is no "best" weapon in DF.
All have drawbacks and pros. So yes, have a mix of weapons on hand. The arena is your friend. Making an arena in Fortress mode is better.
*Also I would recommend you using larger squads with multiple orders, instead of more small squads with single orders. Unless you like micromanage - smaller squads allow for more control, with more effort on the initial set up and maintainance after casualties.
When I use units with identical soldiers, I tend to run into problems like a flying enemy that my axedwarves can't reach, or a super-fast capybara that they will never catch up with unless someone shoots it with a crossbow.
Which is why I have the 3 squad system - crossbow dwarves can be stationed besides 3x speardwarves, axedwarves and swordsdwarves. If more/less are needed, their number can be increased squad by squad.
What if the unit of archers is stationed on the other side of the map?
Why would you do that?
What if a squad of archers gets ambushed without any melee support?
See above. Failing that, it's a good idea to train your marksdwarves to use a melee weapon other than the crossbow. All of my marksdwarves carry a weapon of their choice or else a battle axe.
Any tips for training marksdwarves and then setting up a tower/base for them somewhere? I tried to train them before but it seems as though there's no easy way to do it.
There isn't, best thing you can do is arm them with crossbows and send them to kill capybaras and such. Or give them hunting instead.
Trying to make them train is like playing Dwarf Fortress inside Dwarf Fortress
Mine train just fine ^_^
Failing that, find something for them to kill. Zombies are a good source.