You knew this was coming, I knew this was coming, let's just try to get through this.
On a more serious note, Once 7th drops, I WILL be doing a review of everyone's favourite spehs army, but only when I have a better idea of what seventh will mean for the army. Once that has dropped, I may also finally do a true review of the tau codex, although I'm not sure if that will be worthwhile at that time.
What kind of tank?
This is a hard one. Any leman russ will constitute a substantial portion of your list, and taking 2-3 of them for a tank commander usually means they'll form the centrepiece of your army. What I suggest is either build your army around the tanks, or build your army less about 180~200 points per tank you'll wanna field. When you've built your army, check what it needs and build your lemons to that role.
The Tank Types
The Annihilator is not in the actual codex, but it is worth a mention. The Annihilator packs a twin-linked lascannon. Yeah. But, it weighs in at a meagre 130 points, meaning if you need 'dedicated anti-tank' but don't have the 35 points + extra tank it takes to get a vanquisher with a commander, the Annihilator is your man. Also works reasonably well against monsters and heavy infantry. If you wanna run them, the best way to do so is to run two of them alongside Pask (in a vanquisher), with lascannon and multi-melta sponsons. You're packing 12 S8/9 shots with an AP of 1 or 2, making that a deadly unit for enemy heavy infantry and most armour. Of course, being relatively cheap armour, they also perform roving alone.
The Demolisher cannon is a S10AP2 Large blast mounted on an AV14/13/11 shell. While at first knick, having AV14/13 should mean that the demolisher doesn't work too poorly, you really feel its short range with only 6" movements. On the plus side, until its cannon or the chassis is gone. Because the Demolisher is only as dangerous as its main cannon, I don't suggest sponsons, however, one could get away with heavy bolter sponsons for making the trip to the enemy line - having a little more S5 packed in there for so few points could do well. The Demolisher works surprisingly well when tripled up in a tank commander - note I don't suggest Pask because he doesn't really do very much for the demolisher here. The reason for this is the flatout move that the tank commander can order. This allows the Demolisher to increase his movement speed by a pessimistic average of 150%, meaning he can make ground in four turns instead of one. Of course, doing this, you're dropping 3 S10AP2 blasts and that's about it, and it cost you a fortune to do so.
The Demolisher cannon itself is decptive. Being S10AP2 it seems like a pinnacle anti-tank weapon, but this is not true. While it will knock the living bejesus out of almost any armour it comes into contact with, it can only do at most, 1 hull point. Being a large blast, people have this misconception that it will knock seven or 8 targets, but this simply isn't true. If your opponent is that afraid of the S10AP2 and it gets the chance to shoot, they will spread out to limit casualty potential. Still, The Demolisher remains a fantastic tool when fighting buildings, and an even better tool for knocking out T4/5. Unfortunately for the demolisher, it is fantastic VS. T4/5, making it hard to utilise it Vs. T6. T6 means it can only do at most, one wound and this simply isn't enough.
The Eradicator is a S6AP4 Large Blast that ignores cover mounted on an AV14/13/10 shell, it is the cheapest AV14 in the game. The Eradicator is very, very comparable to the similar wyvern. The Wyvern does more damage for its points, against the same targets, but the Eradicator is a harder takedown. This means you can reliably expect 5 or so blasts per game, whereas you can only expect three salvos at the very most. The Eradicator works well with heavy bolters, and one is usually enough. being a large blast, a tank commander is wasteful, so I suggest that they remain as lone operators. The Eradicator has a place in 40k, but it's true that it would be better suited to a different army.
The Exterminator packs a quad-gun without skyfire, and is reasonably cheap at 130 points. S7 makes for a versatile main cannon, when comboed with heavy bolters it becomes a reliable anti-light tank and infantry machine. Its decent range gives it an edge over the punisher against all the punisher's targets, and the price difference seals this difference. Exterminatorss work well as bodyguards for better tanks like Vanquishers, but can also perform alone. Having a squad of more than two exterminators is probably wasteful.
S7 is deceptive. The internet has a raging hardon for this mediocre statline, but S7 Struggles to perform in many roles. People profess its strength against AV12, but this simply isn't the case. While I believe it can be pressed into that role, the S7 profile works best against AV11 and T6 or worse. This makes heavy bolters ideal for it. Also worth noting is that this overzealous faith in S7AP4 permeates many player's minds, meaning most opponents will shoot at exterminators very readily.
The Executioner packs 3 plasma cannons, and is overpriced at 185 points to maximise its tools. Because of this huge price and the executioner's potency at killing itself, running 3 with Pask is pretty much the only way to run them. Preferred enemy means they should only lose a hull point every five or so turns. You basically have to invest in a squadron of these guys to make them effective. For that reason, I don't feel they're worth wasting half your army on.
The MBT is relatively cheap and packs a S8AP3 large blast. This has better hitting range than the demolisher, but otherwise suffers from the same problems the demolisher does. It's also not nearly as versatile or worthwhile. If you don't like exterminators and don't mind shelling out, two of these make a half (note half) decent bodyguard. Unless the ordnance typing is dropped for heavy, the battle cannon will never be worth the time.
The Punisher Shares some of the Exterminator's weakness. It suffers from people praying at its alter, while usually its short range and relatively weak weaponry limit its effectiveness. You're paying 140 points for a semi-dangerous weapon that knocks very light infantry around for a throw, then gets knocked out.
The punisher is most effective when manned by Pask. This works fantastically well when run alongside a pair of bodyguards that aren't packing twin-linked main cannons. I suggest bringing 3 punishers for Pask. Run the squad up the field until he hits home, then order his squad to split fire. He can deal with almost any armour you throw at him - he should reasonably do 1.72~5.08 hull points to a land raider or rather AV14, averaging 3.4 hull points when shooting. Anything less than AV14, and any Toughness character he should shred a wound or two with ease, averaging 1.7 to 5.2 wounds when fighting anything better than T6. While the punisher is certainly versatile in Pasks' hands, don't overestimate how effective it is. Let your opponent make that mistake for you.
The Vanquisher is Armoured Superiority in a name. Its main cannon can go toe to toe with an orbital bombardment for final damage, and can make him an extremely dangerous opponent for any enemy armour. About the only thing it lacks in terms of anti-vehicle is skyfire/interceptor. Of course, the vanquisher is not without its faults. It needs to be well supported, and works best on a tank commander which necessitates it being fielded alongside a squadron. Fielding many more than one of them begins to become inefficient as it's usually enough to get the job done at close range with a pair of multi-meltas. 3 Vanquishers works without a commander, but it definitely needs to have prescience behind it. With prescience, the pack of vanquishers can potentially flatten even superheavies with relative ease. Also worth noting is if taken with a commander, the vanquisher is shot by a character, and thus has precision shots on 6's. Remember this, as instant-deathing specific targets can be massive.
My Personal favourite from a gameplay, aesthetic and fluff perspective will always be the vanquisher. IT packs a tonne of heat and really embodies what a tank-hunter should be. I know it's not the best tanke, but I can't help myself when it comes to saying nice things about it.