For anti-armor, go tank heavy. If I'm against an enemy with a lot of armored vehicles or weapons, I'll break out my 2 Dreadnaughts, Predator, a Vindicator, and Defiler.
Anti-personnel is a bit different. A dread isn't really needed for this, although having one go psycho and mow down scores of bugs is quite amusing
. Vindicators are great due to their high strength blast when dealing with MEQ, but their range makes them easy for long range enemies to pick off. Defilers are also good for this situation. You can also set a Predator up for anti-infantry, and they will do quite well.
As far as troops, in both cases, Terminators are always well loved. The only problem is, they're expensive and a unit of anything rending will kill them. 5+ Inv is nice, but unlikely you'll get it. Always have at least one reaper autocannon, the rest I tool out with combi-weapons.
I don't really use Havocs, and with the ability to have 2 special weapons in a squad, they're not really needed. I'll rarely use a flamer, depending mostly on the enemy. Bugs and IG, flamers are great. MEQ, not so much.
Meltas can be effective, but the problem is you have to be within 12" to use it, rendering it in most of my games useless until the last rounds. Plasma guns... A++! Yes, you can overheat, but you still get your armor save, so it's not AS bad. I think in all the games I've played, I've only lost 2 marines to Overheating. Same range as the rest of your squad, and can deal out death to most anything.
I exclusively use Plague Marines. No, it's not much variety, but it's tried and true. Death Guard is an attrition army. You use terrain to your advantage and force your enemy to come to you, and you're almost guaranteed to win. Of course, don't do this with Tau or some tooled armies.
For armies excelling in range, I recommend mostly infantry, if not all. This forces heavy gun armies like Tau to pick and choose troops with rail guns and such, making the weapon a waste in the long run. Use your terrain to your advantage and bring the fight to them. Even if you get within 24" with your infantry, Deep Strike Terminators in the enemy's rear is usually devastating and can send their army into... Chaos.