If we're going by all books, DH or otherwise: You're gonna need some strength, or the right mutations to really take advantage of this though. This is for a Techpriest, using the newest stuff; it worked fine before Integral components but they truly are delicious now. Get a high quality EPC, and you'll save on "good" quality across the board. Don't need to tell you about motion predictors, spoor targeters, the new OW version Targeters and anything else you can fit on there is obviously "try to get it if GM allows that one" for all of them: upgrade your stuff.
Sidearm: Ripper Pistol with Maglev-Impeller. A harsh piece of work for a simple SP weapon. If you don't like or can't access that, might I suggest the Carnodon, instead focused on single-shot [so red-dot instead]? It'll do right by you.
Melee: Focusing on dodge here, with the packs or wing mutations or whatever, leave melee to your raptor buddies. So instead of a parrying counterstrike monster: Try out that new, flexible Coil-Whip. One of the best un-parriables, for when one of those khornites gets a little in your face. You don't want HIM getting those free counters or avoiding your hits with WS now do you? Again, Integral, Monowhip. Shocking at -30 off a swift attack? yes please. Added bonus: the Ripper makes a fine other-hand for this.
Primary [but less than big-social-function-friendly]: Integral M41 Multilaser… OR an Autocannon. Both provide light to moderate Antivehicle levels of damage given half a chance, but there's just something about a Tech Priest's "servo-harness" replacing all its stuff with the four barrels of an MIU-linked Reaper-Auto that turns heads all around.
By heads I mean turrets.
By turrets I mean those big ones on vehicles.
By vehicles I mean Baneblades.
Do be careful with those things. Once you're visibly a comparatively squishy-looking threat to tanks, you just topped priority lists.
One more thing: If you need just one weapon, and you're not sure what it is, and you're not certain what you'll be facing, but you're pretty sure you're going to want to be able to deal with lots of things in lots of situations pretty damn well, if not "ideally" like the guy with six guns for each occasion? Get yourself a Pulse Rifle. Short of Tank or Great-Unclean-One hunting[chimera or sentinel you can try it if you've got good dice and mighty shot], you'll never go wrong with Pulse… you could just technically have gone more-right with other weapons for that specific situation.
Unless of course you're a mekboy. Then you should be dragging a machinegun around that puts out 3d10+5orhigher Type E or X unstable twin-linked Storm shots off a ridiculous RoF with enough bonuses to almost put you on par with a dedicated stormtrooper's accuracy there. OR taping a burna in sword-mode to a burna in burna-mode so you can have your power-blade and eat it too.