Build orders are not very easy to explain in Knights and Merchants. Unlike Age of Kings, for example, where the maps are all very similar (with minor differences), maps in Knights and Merchants will greatly determine how to play. This inconsistency is why players must practice often in order to truly understand the game's economy in accordance with what resources are given and available at the start of a game.
The strongest unit combination, like dicsoupcan said, is swordfighters and bowmen. I'll give you my strategy that I use to make that army, but keep in mind that it changes slightly depending on the map, and to compensate for my many mistakes. For example, on the most popular map, Cursed Ravine, you start with only 50 timber. I would construct a 2nd woodcutter and a 2nd sawmill slightly earlier there then on maps where you start with more than 50 timber.
My strategy:
Phase 1: Block all wares in the storehouse except for food, gold, corn, pigs, and maybe iron ore. Give the school a hotkey once it is built. Train 2 laborers then 4 serfs. Then train 1 serf -> 1 laborer combination (of course, making room for other citizens, such as farmers woodcutters, etc., when needed). A woodcutter must be built as fast as possible so it is built first. Once the woodcutter is built, I build 2-4 quarries depending on how many stones I have left in the storehouse. If I have 40, then only build two. If I have 30, then I build three, 20 then four. (Five quarries are only needed if gold is very far away and few stones are given at start.) While my quarries are being built, the sawmill must also be built (make the sawmill as fast as possible). Also, extra laborers should be making a road towards the gold. Once the sawmill blueprint is placed, I make a 3 serf -> one laborer combo in the school. Phase summery: build sawmill as fast as possible
Phase 2:
Once my sawmill is built, make another 1-2 woodcutters. I build a farm in a way so that the road for the farms is heading in the direction of either my gold or iron (to save time); preferably gold. Also, make a road to the iron if needed. If I'm at the gold already, then I build a gold mine+coal mine before the farm (still make the farm if extra laborers are available). make another 1-2 woodcutters. At some point, you will reach =<15 timber. Build another sawmill at this point. Phase summery: Build a gold mine, farm, and iron mine as fast as possible. You should have about 35 serfs and 18 laborers at the end of this phase.
Phase 3a:
If you have 15 or more gold, then the second goldmine + metallurgist can wait. Build a iron smelter quickly. Once the iron smelter is done, build an armory smithy. Once the armory smithy is done, build another iron mine, another iron smithy, a weapon smithy, and another armory smithy. Build a 3rd iron mine and iron smithy once all that is done. Build a 4th iron mine 5-10 minutes after the 3rd if possible.
Phase 3b:
During the iron production, you should also be focused on food and leather production at the same time. Phase 3a and 3b and 3c should be done at the same time.
Build a 2nd farm. Once the corn is almost ready to cut, build a swine farm. Build a 3rd farm. After, build a 2nd swine farm. Depending on how much food you have, build fisherman huts as needed and as practical (don't make three fishermen for 60 fish, for example). If there is not enough fish, then I build a 4th farm and mill + bakery. Once the first pig is killed, build a tannery. A butcher should be built once needed. Once the tannery is built, build a armory workshop, and another after that. Build enough farms for three swine farms (6 if no mill, seven is a mill is needed). A 4th swine farm and 7-8 farms can be built if there is enough peacetime left.
Phase 3c:
Your wood production should reach at least 8 woodcutters at the end of your base building; 10 is much better, though. Once you reach 6 woodcutters that are almost ready to cut, build a 3rd sawmill. Once the third sawmill is built, build a weapons workshop if you have timber to spare. Make only bows but switch to axes if you won't have enough leather + have extra gold. At 7 woodcutters, build a 4th sawmill and 2nd weapons workshop. A 5th sawmill and 3-4 weapons workshops are very nice for lategame spam.
Phase summery: Get weapon production. Keep in mind that you should have at least one coal mine per building that uses coal. Don't run out of gold, food, or timber either. Make a barracks and 2nd school, and towers once needed. Also, remember to build 5-6 quarries if you start running low on stone.
That was a lot of writing...I said more than I wanted to. But keep in mind that the best way to learn is to watch replays of the best players, and actually play yourself
Disco's points were also good.