Destroying old cold war tanks like T-55 or T-72 isn’t that hard, mostly since even the more basic RPG 7 versions can destroy them by penetration, even if they hit them in the front. However, what happens with M1A2 Abrams, T90AM or the new Armata, Leopard 2 and so on, rpg’s and even most at guided missiles (well except the Javelin) can’t penetrate them, so what do you do against them? I suppose that you can hit the tracks and disable them from moving and thus score a ,mobility kill", but the tank will still be able to shoot back, of course you can also plant a IED or something, but you can’t destroy all tanks that way, maybe one if you’re lucky, so that’s not the answer either, so what do you actually do?
You take a really big gatling gun and build a plane around it. One of the Warthog’s primary missions when it was designed was to kill tanks.
A number of rocket-propelled grenades can defeat Abrams armor at various points. This includes RPG-7 and RPG-29. More importantly, armor is not uniform all over the tank. The front glacis is usually the most heavily armored point, followed by the sides, and then the rear and top. Most tanks are notoriously weak on top, partly because they need to shave weight off the part least likely to be struck and partly because of the non-negotiable need for an access hatch.
Need answer fast? 
The A-10’s gun isn’t effective on any tanks made since the 70s. Its tank killing ability comes from the Maverick missiles it carries.
Once immobilized, it’s no longer a tank but a petty fortress. Sure, it can still shoot, but even if its’ armor is impenetrable to your weapons, metal is a hell of a heat conductor… just dump a ton of gasoline on it and set it on fire!
Even the top armor?
Yeah… That’s why I stopped where I did.
Yes. I was trying to find cites, but strangely enough there are no sources I could find on the internet about the thickness of T-72 top armor. Going from memory, later versions of the T-72 have around 80 equivalent RHA against kinetic penetrators on the top, but I can’t find a source to back that up. GAU-8 penetrates 70mm at 500m and 40mm at 1000m. Typical attack runs would come in at a slant rather than straight down, which significantly impedes the potential armor penetration (coming in at 30 degrees off perpendicular effectively doubles armor).
Later tanks were designed with the A-10 in mind, and have sufficient top armor not to be vulnerable. Which isn’t to say that an A-10 couldn’t mess up the treads, sensors, optics, ERA, etc. of a tank, so an A-10 attack could score a mobility and/or firepower kill, but they generally engage tanks with their maverick missiles. The gun is reserved for lighter vehicles than tanks.
I take your question to ask what means have a good probability of getting a complete kill on a modern tank.
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A HEAT or EFP warhead from a top attack munition (missile, mortar, artillery) Plunging fire - Wikipedia
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A height advantage to use a projectile from a dumb rocket launcher/recoilless rifle and hit the top (like Chechens did in Grozny)
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A guided bomb to hit the tank like the GBU-53 GBU-39 Small Diameter Bomb - Wikipedia
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Another tank’s main gun using ammo optimized for MBT killing. It’s not sure right now which of the latest armor or the latest APFSDS has the upper hand Armour-piercing fin-stabilized discarding sabot - Wikipedia
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A ridiculous amount of explosives and waiting until the tank gets close enough.
With all those facts in mind, let’s have a practical example, for example Turkey has 350 Leopard 2A4’s and most Balkan countries have T-72’s and their local variants and mostly ,meh quality" AT systems, really weak air forces,etc. , if in an unrealistic scenario Turkey decided to occupy Bulgaria, Serbia, Croatia and the rest of the Balkans (not including Greece which also has the same tanks) , would she be able to do that because of those tanks or would the Balkan countries (united) somehow manage to stop the Leopards, even though they don’t have any single piece of equipment that could really beat the Leopard? Of course many others factors could affect the scenario, mostly population, Turkey has a huge population, so let’s say that the Balkan countries have the same population as Turkey, both armies have equal number of soldiers, equipment, are equally trained, the only difference being in the fact that Turkey has 350 Leopards, so could they win the war?
The Leopard may have superior sighting but the Russian 125mm gun is more powerful than the Rheinmetall 120mm, at least at close range.
There are very few wonder weapons and tanks aren’t among them. The tanks might be a significant asset but they wouldn’t turn the war one way or another.
Israel has great tanks yet it still had trouble in Lebanon in 2006.
The terrain of the Balkans would require the tanks to be accompanied by infantry and combat engineers. The infantry/CE would be vulnerable and with little/ineffective infantry/CE support, the tanks would be vulnerable, even to “meh quality” AT systems.
So, having really great tanks would help but as to whether Turkey would win the war: We dunno.
Modern tanks, especially the T-90MS have composite armor, with Explosive reactive armor on top of that. Many areas are also caged (a grill like shield) that causes premature detonation of the incoming rounds. Plus they have angled the armor to about 70 degrees in many places which alone pretty much guaranties a bounce from incoming rounds. Add to that they incorporate a spall liner, which protect the crew from pieces of the armor flying around in the turret if it is hit.
The new Armata will have all that plus an active protection system that uses a very short wave length radar to intercept incoming rounds and defeat them. The anti-aircraft machine gun is also radar guided on this tank, plus all kinds of other magic stuff I really don’t fully comprehend.
The tank is said to be able to track 40 airborne targets and almost as many ground targets simultaneously. And fire ATGM (missiles) that are also effective against helicopters and low flying aircraft.
The Armata is an autoloader, with 22 rounds in the magazine and another 18 in the turret. Firing rate is said to be around 8 rounds a minute. Rounds are laser targeted and guided. The barrel of the canon has a laser system that reads and compensates for bend in the barrel due to temperature or wear thus increasing its accuracy.
Russia abandoned the turbine engine with the T-80 I think. They actually had a turbine powered tank before the US, and have abandoned it due to logistic problems with respect to getting a special fuel to the battlefield. They have returned to turbocharged diesel engines.
Interestingly, if you look at the T-14 (Armata) there is no fume extractor on the barrel. (that knuckle typically seen in the middle of a tank barrel) That extractor prevented poison fumes from the burning powder from entering the turret and poisoning the crew. The T-14 has a crew of three, sitting in a titanium surround, in the front of the tank, so there is no one in the turret.
T-14 Armata picture.
Seriously, Tanks have come a long way as has anti-tank weapons. But this Armata is certainly one bad MoFo that the US needs to take seriously.
I imagine a laser guided 500 lb bomb from a fast mover would also be a kill on any modern tank. Surely their top armour couldn’t take that?
You nuke it from orbit. It’s the only way to be sure. 
Thin top armor; many antitank missiles fire downward projectiles.
Having a cornucopia of armor isn’t just expensive, it’s a potential danger. Ceramic armor can stop a high velocity tungsten round but could crack when hit by a thick-walled AT or a solid shot. Reactive armor is good against a shaped charge rocket but useless for anything else. Nothing beats thick, solid, extremely hard steel or even uranium. I reckon armor made from cpm s90v or cpm M-4 steel will be better and cheaper than uranium. And it’s potentially cheaper to manufacture than casting, since it entails powder sintering.
Addendum: cpm M-4 is not a powder steel. My bad. the Japanese zdp-189 is. It has a “tough” hardness of rc63, but could go to as high as rc 67. For reference, a standard file has a rc of 62.
The front armor of a T-72 could defeat a 105mm US tank round in the past. That’s one of the reasons both sides went to the composite armor and the ERA wrap. I see a bigger problem of convincing troops to fight along side a MBT with ERA, knowing that the armor explodes when hit (albeit not by small arms fire). I’d stay as far from a tank as I could if I was infantry.
I don’t know much about military stuff, but I guess a Javalin missile penetrating from the top where the armor is weak should be able to take out the crew compartment at least. As long as you hit the tank from the right angle where it’s armor is weakest, you don’t need the most advance missile.