There's no absolute 'best' unit since there are so many variables to consider.
- Cost/effectiveness?
- What terrain?
- Available in what era?
- What patch version? (I assume 1.2/1.3 without retrofit mod (JHI suck when patched))
- Vs what units?
- Defence or offence?
Just to name a
few...
Of course this is all very boring to consider since it would mean there's no absoute answer to your question. Thats why you see so many 'opinions' on the board.
Personally I consider the following units the best:
-
Venetian Heavy Infantry
Available really early and are ridiculously strong with their hammers of doom. Only downside is their low morale.
-
Hashishim
2 hit points, highly trained. Great at everything (especially defending walls).
-
Dismounted Dvor
Awesome archer unit with really strong melee and high defence. Not best in direct assault but add some tactics into the equation and these guys can outperform almost everyone.
-
Aventuros
Best spearman. Their amazing spearwall can do wonders in a defensive situation
-
Dismounted Conquistadores
Best stats of any 'normal' shield/weapon infantry. This is probably the unit you're looking for. However they are hard to get, *very* late era and a bit boring to use since they don't need any real tactics to be effective.
-
Naffatun
Seriously, they thrown burning oil on their enemies. How can ANY list be complete without Naffatun in it...

As a sidenote, they also happen to own, even elephants, when used correctly (mostly in small flanking groups).
If you're more specific on some of the variables I've mentioned you can get a more fitting answer though.
- Soltaro