Land Raider
Sometimes the only solution to a problem is a complete lack of anything resembling subtlety.
A Rhino Transport grown several orders of magnitude too large, the mighty Land Raider is the biggest available tank that Space Marines and Chaos Marines can field. Its firepower is incredible, its armor betrays no weakpoints, and it has maximum armor (14) for every side. It can also transport Terminators.
The Land Raider has always been a somewhat insane beast, owing to its large weapon load and the fact that only part of this load can usually fire at a given time, making it so that it's of questionable value - this is because there are almost always better choices to do what the Land Raider can do for cost - Predators generally do better at vehicle busting or infantry killing, and the main use of the Land Raider is as a transport vehicle for Terminators and getting them right into the heart of the battle, which is kind of lopsided with its heavy armament.
Even with this, the Land Raider is still something of a good combat vehicle, if only because of the changes to the 5th edition vehicle weaponry rules. Stationary, the Land Raider can fire off all its weaponry in a salvo, which is especially good for the Chaos version, which can mount a Havoc Launcher to add a little extra infantry-raping punch. The Gray Knight version is also notable (Psycannon Bolts help this thing a lot) since it's pretty much the only heavy tank Daemonhunters get.
Make no mistake. This thing may be inefficient, but it's a daunting foe if it sticks around, whether it has troops in it or not. AV14 all around makes it a tough nut to crack without dedicated anti-armor weapons, and the Land Raider still is likely to survive the opening salvo.
Variants and Such
The Land Raider has a ton of variants. Some of the more noteworthy ones are listed below.
Godhammer Pattern The basic one has two twin-linked Lacannons in Sponsons and a twin-linked Heavy Bolter. It cranks out a lot of firepower, but this is pretty much at odds with what the damned thing is intended to be (a heavy transport). It's very flexible, but generally inefficient. The Chaos version's a little better at acting as a mobile strongpoint due to its Havoc Launcher, though it's still kind of inefficient.
Crusader Pattern Replaces the Lascannons with Hurricane Bolters and the Bolters with Assault Cannons. Designed to get assault troops right into the thick of it. Built in extra armor and frag assault launchers.
Redeemer Pattern Keeps the twin-linked Assault Cannon of the Crusader, but replaces the side-mounted Hurricane Bolters with a pair of nasty AP3 flamers with S6. Devastating to infantry but will need a Multi-Melta for handling tanks. It's cheaper than the other Land Raiders by about 10 points but cannot transport Terminators.
Ares Pattern (Chapter Approved) Decides to say "Fuck that shit" regarding transporting and seeks to become the most destructive linebreaker around. No transport capability, and costs about 50 points more than the standard Land Raider, but you get what you pay for; Twin-Linked Heavy Flamer Sponsons, Twin-Linked Assault Cannons and Demolisher Cannon make this thing filthy rape to anything with the misfortune of getting anywhere near it.