I say Dath Vader or any Sith would be a decisive win against most of the Imperium of Man. The Grey Knights may stop psychic powers and warp, but the force isn't either of those. Its midichlorians, which if memory recalls are cells that can control the pervasive level of ENERGY, not a warp or psychic power. On top of that lightsabers are highly stables bolts of plasma, being to the point of touching anything it melts, cuts, and cleans a cut right through. That being said, not many kinds of armor could survive it. I don't think the Imperium has the mineral, that vibroblades were made out of. (Sorry I can't remember the name.)
As for ship versus ship. An Acclamator Troop Transport outputs somewhere around 300 million GigaWatts. And thats only with 24 guns. Sheilding on that can take almost 700 trillion GW before thats even broken. And then it has the 34' hull, which was made from a form of steel that was almost incrediably powerful. I forget the name of that too. And thats just a troop transport. A Imperial Class Star Destroyer is almost 10 times larger then that, so by standards it could be up to 10 times stronger. (Most of this info was taken from a canon book.)
Land vehicles also contain sheild generators, at some points. Vehciles like the AT-AT can have sheild generators built into the hull, making it an already formidable force against anything to making it a monster able to deal and dish the immense damage. (AT-AT's fire roughly I believe 150 million GW of energy in bolt for roughly 1 every 4 seconds.) Now say AT-ST can deal a lot less then that obviously as its a fast scout vehicle. It has much lighter guns but its interchangable with different turret types allowing Anti-Air/Vehicle with the torpedoes they fire, or Anti-Infintry with energy bolts.
Troopers are actually an elite infantry force. The 501st is a seasoned, WELL seasoned battle regiment from the beggining of the clone wars. They have experiance fighting in space and fighting against droids or humans. They have far superior technology and armor then most give them credit for too. Hell, the only reason why they would "miss" in the movies is because they were shooting at the heros. If you look in the movies they are actually very accurate. They even show advanced tactics, like in the beginning of ANH, they boarded the ship almost immediatly and only a couple got cut down due to surprise attack, but they dominated the rebels shortly there after. Again about the movies and ewoks they were never actually beaten. As someone has already said the 501st continued to be a major pain in the ass for The New Republic. Storm Troopers obviously have incrediably fast reflexes as well noted on Endor during the speederbike scene. A normal human would have a hard time navigating that as well as they did. The only reason Luke didn't crash is he is a jedi. Leia crashed pretty fast but obviously didn't die because she is a hero.
The Storm Trooper technology is also on par with many things. Their blaster is incrediably powerful as it was fire plasma bolts, most notably drawn from the fact it uses a plasma gas cartridge for 500 shots. The E-11 even had many different variations from a longer barrel for longer range and a sniper rifle, to even a carbine. Storm Troopers also had a side arm (Not seen in movies canon books and shows.) the SE-14r, which was short range obviously but again fired plasma bolts as its implied most SW weaponry fires. Elite Storm Troopers also used a DLT-19 Heavy Blaster rifle. Which could hold many more shots, fired at an increased pace, and had far more accuracy. All Storm Troopers also had a thermal detonation core on their back, which had a unique input code that only the Storm Trooper knew. And even then, it was completely disguised in the back of the armor requiring special ways to get to it.
Armor is another big thing many people look over. Many of the Storm Troopers "killed" probably werent. The Plastoid armor is capable of take bolts and impact, as well as protecting against concussion and explosions, and even the enviorment. Sure a direct hit of plasma could cut through the armor, but it wouldn't actually kill them unless it was a head or direct heart shot because it would barely graze against the skin due to the compositiong of the armor. Very often the armor also had holigraphic displays in the helmet, wrist comms, and resonaters. Storm Troopers could also be outfitted with packs (As seen on SandTroopers.) that had water, food, rations, more ammo, tents, and even Dwarf Spider Droids for support fighting.
EVEN MORE SO, there are specialized units of Storm Troopers. Ranging from Aquatic fighting to Zero-G Assault Troopers. There are many ranges including Force Sensative Troopers which had limited use of the force, and to detect. There are even NovaTroopers, who specalize in shock and awe tactics of boarding enemy ships and having layers of strips and armor, making them bulky, as well as behind enhanced by neuro making them tougher and faster capable of taking ships, infiltrating by force or simply used to force the enemy to surrendur from sheer fire power. You also have Shadowtroopers, which have built in stealth units and specalize in hand to hand combat as well as silent killing. You have Imperial Marines, Incinerator Troopers, Heavy Troopers, Dewback Troopers (They are mounted units for quick assaults.), Field Troopers who are veterans from the Clone Wars and have incrediable amounts of experiance, Magma Troopers, EVO Troopers, Shock Troopers, Storm Commandos.
And of course you can't forget DarkTroopers which are Endoskeltons of clone/droid cyborgs on the indside, resistant to heat, plasma, lightsabers, a whole list. They are even specalized themselves the most notable ones being Purge Troopers who were Jedi killers, I'm positive those could take on the guard. You then also have the Three Phases of Dark Troopers, Phase 1 CQB using a sheild and a vibro blade. Phase 2 more precise storm troopers, with heavy repeating blasters and assault cannons plus jetpacks. Then the final Phase 3, is based almost entirley on Boba and Jango Fett, using Jetpacks, rockets, flamethrowers, blasters, and cannons. All being mounted on the exoskeleton leaving the hands free.
Adding even more to the pile as my final two sense (I may quote myself and post more if I remember anything else.), the Empire is highly mobile as they use Hyper Speed, not warp speed. So they can deploy much faster then the Imperium. And they have continuous clone resources, as well as recruits joing into the Empire after the formation of the New Republic. You don't give the Storm Troopers enough credit. They are an incrediably powerful fighting force and could easily give the Imperium a run for their money.
And I didn't even start on their ships, or the Sith. (Keep in mind, their are also Sith/Storm Trooper hybrids. Just something to think about.)
So my whole statement here. Is that accounting for everything the Empire has to offer, (Death Star with held at this momment.), the Empire could win. Not a total crush, it would be a long and hard fight. But they could win.
