@freezy, I'm not sure if you're already following this thread, but I think you should.
My tentative list:
1) TANK DESTROYERS!!!
This unit is a big frustration unit for me. Potentially, it's a great unit. They should be mobile defensive units, capable of quickly closing gaps when the enemy tries to blitz you with an LT army. Its combat stats reflect this role: great on defense against armor, not too good at anything else. Exactly the role they were used for in reality. A truly GREAT unit!
But now, the frustration part. It lacks one vital stat to perform this role: speed. It can barely keep up with towed units, and rushing to a threatened province is right out of the question. This condemns it to a static role, defending only a single province. But... ah, wait... don't we alrady have a unit for this? Ah yes, AT guns! They don't use oil, don't require infrastructure, are faster and cheaper to build... so, you need the TD's like... never!
Roko's advice: increase speed to halfway between MT and LT.
The speed problem of this unit was overlooked in the last balancing round... now we've moved on another half year, and the TD still isn't used by anyone in any significant numbers, for exactly this problem. @freezy, PLEASE don't forget a drastic speed increase for this unit, and if possible, sooner than a general rebalancing round! Don't make us wait another half year before doing something about it!
2) NAVAL BOMBERS!!!
It is not true, this isn't really one of my least used units. I try again and again, but I just can't get them to work. And in a long history of playing this game, I have seen only ONE person to operate them with great effect. I'm not talking about their sub scout role now, it is tentatively useful for that; I'm talking about its combat role against surface ships.
Again, the combat stats aren't really bad. A full stack of them could engage a medium-sized fleet and win (well... barely). There's two problems with this though.
First, there is a HUGE investment involved in both time and resources to GET this stack. The research cost is one of the most expensive around, L2 air bases are always scarce and competing for a lot of other stuff (tacs and L2 rockets, both VERY useful units), and unit cost is also high.
Second, you need to find and engage your opponent. For an empire that can afford a full-sized stack, the coast line is probably long and the enemy navy can pop up in many places. Now the NB's problem is its range. Air bases are expensive, and likely tailored to fighter range. NB's range is below that, so it usually just can't fly to the place where it needs to be. More AB's you say? Sure. We all have endless amounts of res lying about, don't we?
This unit has a third problem. Let's say we invested all these huge amounts of res and effort, and have a full NB stack now, escorting fighters, an air base network tailored to them, the whole shabang. Sadly, the enemy doesn't come by sea. You have this huge big expensive NB stack, and can't use it on anything. Period. Completely useless in any other role. Yeah, shit happens.
Roko's advice: I think this units needs several GENERAL boosts rather than one stat increase. Upgrade range to fighters of the same level; decrease research cost by AT LEAST 50% (actually I'd say 75%) and time by 33%, then decrease unit cost by 20%.
3) INFANTRY!!!
This is also not true; they aren't really one of my least used units, simply because of the bunch you get when you are born; then I build another round of them as they are the only practical units you can build on the first day. But after that... no, I never build them. And it's not because I think they are bad... it's because they are expensive.
What, infantry? Expensive? Hell, yes! Contrary to any war ever fought in history, infantry is one of the most expensive units around. In the early game, manpower is scarce and vital, and a staggering 1,500 of it makes them more expensive than, say, two light tanks. And when the manpower problem is over, the food problems usually start... so, guess what? Infantry is expensive AGAIN! I'm really fond of the infantry I get at game start, sometimes they survive a very long time and I put them to really good use, but build new ones? No, thanks.
Roko's advice: well, actually, a quite drastic one for consideration... increase all manpower income by 50%, and decrease build time for infantry to 33%. In this way, everyone could build the sexy units without any MP problems; and all the excess of it would be converted to infantry. Yes, the battlefield would be swarming with infantry, like it does in all wars. This would probably raise some more balancing issues... well, lets discuss them. But that's the general idea.
4) ARMORED CARS!!!
Again, you get some at game start; but after that I never build them. Inferior in all respects to LT's.
Roko's advice: specialize them even more: more speed, more visual range, so they're obviously BETTER than LT's in these respects (speed in particular).
5) Medium and heavy tanks.
Heavies were already mentioned a few times, I'd like to extend that to mediums as well. By the time they become available, long-range firepower also becomes abundant (arty, planes, rockets) and melee combat is on the way out. LT's are simply better in the remaining role, land conquest. Well, for my play style, anyway.
6) Some of the usual suspects that should objectively be higher up on this list: militia, strat bombers, nukes, rocket fighters.
PS - if someone asked me a week ago which unit would NEVER be on anyone's list, I'd have said "tactical bombers" without blinking an eye... I was very surprised that they popped up!