Well, I suppose my opinion is pretty invalid since I'm still on Regent but all through my Regent time I found Explorers, Helicopters and Paratroopers to be VERY useful for winning wars.
Explorers use = Fast Paced Border Pillaging and Saving my Border Villages. 1st turn of war I send them out to pillage the main roads to slow their armies approach on my cities. Also I managed to cut off an Iron and Horse (although they had backups) from access via pillaging. I'd assume if you are close to the capitol they could be used to pillage the surrounding roads to end trade.
Paratroopers use = Ever ammassed about 40 Bombers? I did. In basics, I'd be able to bomb a city 'til all of it's guards had died. But they were too far off for average cavs to reach and takeover. So, I started dropping off groups of Paratroopers into Enemy territory, Bomb their units away and move the Paras to take over the city. Make a captured Worker make me an Airfield in my new 8 Square territory and start airlifting better units into that airfield. I managed to capture a central City this way and shipped in 20 Tanks into the center of my Enemies Territory ^^.
Helicopters use = Similar to the Paratroopers, dropping off Infantry into similar positions, was my basic strat 'til I got Mech Infantry and they became obsolete (sp?), forcing me to start creating Paratroopers
- Basically I found valuable uses for 3 of the units elected for being useless, although this is lower levels. I'm sure at Diety etc these uses are cut down significantly.
I don't think there really is a most useless unit though, but alot of UUs just aren't "good enough" to be worthy replacements for the unit they replace. The Bowman has always annoyed me since it triggers such an early GA and Swordsman are better as far as offense goes (and the only reason you'd use a Bowmen for invasion is that it saves you a Spearman escort) and equal in defense, so short of that bombard thing they outrank Bowman [ok, well in Shield Cost and resource requirement to], and they hardly upgrade into anything "hugely" worthwhile (the Longbowmen basically negates that "no escort required" bonus).
3-Man Chariot has never done me any good. They take too long to build and are basically useless the moment you see Jungle or a mountain range (which happens alot ><) that you've yet to road. Although in a 5 Billion y/o map with the minimal possible Jungle I suppose I can see it's potential, I've just never been lucky enough with it :'(.
Medievil Infrantry? I've never ever used these effectively. Longbowmen are easier to access res-wise and that 2 Defense really isn't good enough in the age of Knights, so they tend to need an escort anyway. I'd rather have the bombard bonus to a pathetic defense that I rarely use (due to the Pikeman escort).
Anyone found Carracks useful? On paper they've never looked appealing to me.
The enslavement ability of Man-O-Wars (IMO) makes them worthwhile units, especially with the occasional Barbarian Galley floating about.
F-15 looks like it comes too late and fills no purpose that a Bomber/SAM can't cover.
Guirellas look pretty worthless to me. Defensively, you've got Riflemen if you haven't got Rubber. Offensively, you've got Cavs 'til Tanks come along. Bombard-wise, Artillery ^^. I can't remember it's shield cost though but I know its equal to Infantry, which are more expensive then Cavs and Artillery (right?). Although I suppose it has value in the fact it can do 3 jobs in 1 (Offense / Defense / Defensive Bombardment), even if its doing a below average job in them.
What do Stealth Fighters do that Stealth Bombers and normal Fighters don't? (probs something I didn't notice)
Meh, this is the n00b speaking, I'm sure some ppl here can set me straight on some of the above comments