The Impi is a nice one too. Build horsemen and team impi w/ them and go to war early. I would like the War Chariot, but it's really only useful if mountains aren't nearby and most of the earth is flat. If that's the case, then they work as dirt cheap horsemen. Build a horde of them and just storm cities.
Jag-Wars (Jaguar Warriors) suck unless your neighbor is like a couple squares away and you go to war asap. It takes a good number of them to take out a spearman eating up unit support, so they become great for scouting and that's about it.
F-15 are nice, but they come at a time where you really shouldn't be relying on them (except to intercept bombers/fighter). Your're probably churning armor and mech inf. by couple turns or even each turn by then and can wisk them into enemy territory immediately w/ railroads. Now if they made the stealth fighter a UU...it'd really be late, but I use stealth more than I use fighters, at least in modern age. Late Industrial I have bombers and fighters (send fighters in first, then bombers)
Bowmen/Musketeer units aren't great because of the simple fact that a units offensive power means little if anything when on the defensive. Don't believe it, try attacking calvary, they aren't jack when on the defensive. Attack a longbowmen, they'll be on their back faster than a $2 whore. Since these two units are defense oriented, they shouldn't be used for offense, but they get an offensive bonus.
The 4 defensive given to Cossacks allow them to be stand alone units in their time, but unlike the samurai, they cant be used as city defenders for a long period of time as you should be getting Nationalism soon after. However, considering 1) how powerful calvary is at storming cities and 2) the weak defensive normal calvary has, the Cossack is a nice UU.
Man-o-Wars suck, but that is because the timeframe it enters and the fact that Civ3 naval warfare blows in general. Unless I'm playing an Archipelago, after galleys (to explore) the only ships I build until battleships and transports are galleons (soldier transports).
Already made my pts on Legions. Never played Chinese, so can't talk about how effective or useful it is. Made pts on samurai. Never played as Germans either, I'm usually bored by the time tanks come by though. By then my empire is huge and to be honest, I find having my infantrymen w/ artilery waiting for backup camped outside a city fortified w/ their own infantry more interesting than plowing through w/ tanks, but that's me.
The hoplite is nice too considering how early they're available and how long they are useful once built. Also good for defending that stack of swordsmen/longbowmen when on the offensive and once a city is seized, if has it's defender in place.