Let's get started with the unit that everyone seems to associate with me. The "Marshal Laeroth" Crusader squad, of sorts. For new readers, that is more of a gunline style Crusader squad. What does that look like? Usually its long-range weaponry on a minimum sized squad of 5. That can be lascannons, missile launchers, heavy bolters, or whatever else. I typically would not recommend anything other than lascannons for this role, simply due to the strength, damage, and range of the weapon. This allows the squad to sit back on your objectives, but still be effective in the battle. You would often pair this squad with a Razorback, armed with Twin-Lascannons (or Twin-Heavy Bolters if you are running thin on points), for emergency redeployment purposes. Mid-range weaponry options can include the Multi-Melta or a Grav-Cannon. Both are effective, but I prefer the Grav-Cannon, due to the sheer effectiveness of the weapon against multi-wound units. The downside of this is that fire points no longer exist in transports, so you cannot just sit in your Rhino and shoot out of it. Thus, you have to jump out and shoot...leaving you exposed to your enemy's mid-range firepower. Which would be fine if you could kill everything your shooting at, but typically, you cannot with a single crusader squad. Risky gamble. But if you must, pair it with a Razorback with twin-Assault Cannons so that you have some high efficiency shots coming from multiple angles.
Then you have the mid-sized assault squad that is mounted in either a Rhino or Land Raider Crusader. You will see multiple power weapons on these squads, plus the ability to take close combat weapons to generate additional attacks. Its not a bad idea to pair HQ units with these guys to give yourself force multipliers to ensure maximum damage. Rerolls to hit are critically important here, as they can't be considered dedicated close combat units without it. On that note, I think its important to realize that Crusader squads can never be considered true close combat specialists. They can fulfill the role, but are more of a reactionary force that handles threats as they arrive in battle. Obviously, the more there are in squad, the stronger the unit is. Beyond 10-men, we're forced to take the LRC for mobility. Which isn't a bad thing, since the changes to the smite-wielding units makes the LRC more effective. Just expect a price hike of 21 points when Chapter Approved hits. You can also have the 5-man assault oriented squad that is Razorback based. This unit is meant purely to compliment other units on the board whose dedicated role is close combat.
There is also the foot-slogger unit. This is a full 15-20 marines, walking across the board. While neophytes don't have as good a save, nor leadership, they do bulk up the unit to more durable numbers. If an assault oriented unit like this hits your opponent's lines intact (rare occurrence), it'll do significant damage...especially if you have a Captain/Chaplain tagging along. Alternatively, we have the bolter blob setup. This unit is definitely meant as objective holders, especially in the middle of the board where you can bring your rapid fire weaponry to bare. The downside here is that you are also in your opponent's range. However, I love this setup. Its cheap and effective, with lots of bolter shots and bodies.
So what is the best way to outfit them? Honestly, you are going to get one of the most frustrating answers ever...it depends. It depends on what the rest of your list looks like. I am a fan of the mid-range shooting units in Razorbacks. As well as the longer-range units that I am affectionately known by. But I have also grown fond of our 20-man squads, as they seem to be an effective way of bringing marine bodies to the board, for a reasonable price tag. And in the end, that's what is awesome about our Crusader squads: their versatility.