The most advanced pattern of dreadnought armour available at the outset of the Heresy, the contemptor features a 5+ invulnerable save due to the atomantic shielding and gains the fleet special rule over its normal dreadnought companion. The weapon load outs are slightly different here as well: plenty more paired weaponry is possible, and, if I'm honest: should be taken full advantage of.
Below are a few sample builds, but in each I concentrate on only a single contemptor. Multiple contemptors can be taken as a single force organisation chart slot and deployed together as a talon but then function as a unit in their own right. Clearly there are some synergies to be had and multiple redundancies.
Contemptor dreadnought, 2 multi-meltas, extra armour (185 points)
A vehicle killer. Plonk inside a drop pod if possible and deep strike near its selected target. The extra armour is to help survive vehicle explosion blasts and other incoming firepower should the need arise.
Contemptor dreadnought, 2 twin linked las cannons (225 points)
Another vehicle killer. Or monstrous creature. Blast away from the back lines. Good luck!
Contemptor dreadnought, 2 twin linked auto cannons (190 points)
A good deal of withering firepower to build an anti-infantry dreadnought out of. Probably nicer to take the Kheres cannons (below) instead though.
Contemptor dreadnought, 2 Kheres Pattern assault cannons (205 points)
I regard this one as the de-facto standard build for firepower. Regular dreadnoughts cannot take this option, so one might as well use the advantage of this contemptor option and put it to good use. Heavy 6 rending is really, really significant. Two of them can be devastating. Well worth the points, even on a contemptor with no other upgrades.
Contemptor dreadnought with 2 close combat weapons, each with an inbuilt heavy flamer (195 points)
A classic close combat dreadnought. Replace the heavy flamers for other weapons (plasma blaster, melta gun, graviton gun) to taste. Add on a havoc launcher if you have the points, and place inside a drop pod.
Contemptor dreadnought with 1 heavy conversion beamer, havoc launcher and close combat weapon (225 points)
The conversion beamer and havoc launcher have overlapping ranges (unlike many of the other weapon combinations) and can provide a nice synergy. This guy sits at the back and blasts away - assuming a good line of sight. The close combat weapon is to deal with pesky infiltrators and the like, making for a fully flexible dreadnought role.