I'm torn, I like serpentines - they're the best cannon as far as I'm concerned, the larger ones are a bit over blown with their 'look at me' explosions and the smaller ones look a bit too basic and dull, serpentines are just right for standing there and going 'bang' a lot. They change the way you fight too I find.
Two handed sword units are fun too, again not always especially useful but the HRE's ones in particular look quite jolly.
I'll vote for serpentines I think.

Terikel has proved a popular unit, and faction I think, in the last couple of forum awards.

Inspired by TotalwarLegend's peasant campaign I'm currently trying an England campaign only recruiting billmen - learning that they really do struggle in quite a lot of situations and also that perhaps I should have left Nottingham as a castle so that I could actually recruit the blighters somewhere other than Caen. After I finish that (one way or the other, France is trying to make it the other) I might try an HRE campaign with only two-handed units (allowing for the strapline "you've got two-hand it to them" or similar) say zweihanders, forlorn hope and halberds only.

They're not the best, but Italian cavalry militia have win me a fair number of battles online. Sure, they have bad morale but they have enough armour and crucially charge to make up for it unlike their northern cousins. The enemy completely underestimates them online, and I have a replay in the downloads section where they went almost toe to toe with tsar's guard and with a bit of clever running managed to defeat them and the rest of the army.
Just keep your general close.

Faction wise Teutonic order in kingdoms-the dark feel, the religious mechanics, the dreaded ritterbruder... It felt like you were grim crusaders on a mission

Any words on mods needed?

Sure! can always include it if i lack content(but shouldnt be a problem )

Well, my favourite unit is the Italian Spear Militia. It is one of the few early units that can still be used during the late game. It's reasonably priced, large and has free upkeep in cities, while it as the potential to form the backbone of your army. In fact, such units have saved my ass many times in the campaign.

As for factions, the Kingdom of Jerusalem in Kingdoms is by far my favourite one. Good starting position, good generals, excellent heavy infantry and heavy cavalry and the possibility of strong economy.