As crusader you have only one mobility skill and that if you use the armour.

Best way to counter this would be a personal transporter and "enemy can drop a supply box on death" mod on a item.

So grav gun is good for leveling up, but later i would try to move either into melee (2hand mace looks good) or bolter/autogun playstyle.

Deflect builds with shield/sword are good when enemies are close together, but with the new warzones and void missions, dont find those kind of clustered enemies any longer. On the bigger maps you have groups of 3-4 enemies most of the time. Maybe im spoiled with my psyker, but the crusader really feels kinda slow.

The basic of every ARPG is:

less time in missions=more items

So you need a build that is 

A: quick movement

B: can kill  mobs without taking too much time, that means you kill normal packs in more or less one shot.

Its hard to achive that with a crusader, since you lack of movement. So if you use a range weapon, you will always have trouble beeing quick enough.

You could play a brick tank that is unkillable but you need more than 5min per mission. Thats not efficence at all.

The only range weapon that gives you movement is the autopistol. Other than that, your armour which needs focus and that is only recharged by a fixed time, so you will run out of focus to use your armour boost.

Those things considerd are pushing you into the usage of a  personal transporter (imo the best choice)

Then you can use basically every range weapon you want. For good AOE is the grav gun still the king, but the devs mentioned they might nerf it (tbh that thing is straight out OP) Melta gun is good for burning down stuff.

The heavy bolter could work, if they buffed it.

Other than that its the same as with the rest. Use items that benefit that weapon and your playstyle and you're good to go.