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First, you should probably be using Vincent Girard as your commander for the Challenger series, because he has mobility boosts that really help these unwieldy tanks.  He can also boost health by nearly 15% which is nice on any MBT but especially good on Challengers and Merkavas because they already have large health pools to start with.  Girard can put a Chally over 5K health.  I use Douglas for most of my MBTs, but Vincent for the Challys and Merkavas.

Second, retrofits will depend on both how you plan and what game mode you're playing.  I can only comment on PvE because that's all I play.  PvE is all about doing as much damage as you can as quickly as you can, so retrofits are generally chosen with that in mind.  That's done with gun retrofits and vision retrofits (you can't shoot if you can't see).  In PvE, every vehicle should be using the Improved Gun Breach v2 retrofit which increases rate of fire by 12.5%.  It's the single best retrofit for PvE and short of some really weird corner cases I can't think of any vehicle that doesn't benefit from having it installed.  After that I look at the vehicle's accuracy and aim time.  I prefer accuracy to be below 0.10 and aim time to be below 2.0 seconds (preferably 1.5 seconds).  So if those need adjusting, I use the appropriate retrofits.  Then I'll slot in the visual range enhancements if I still have room.

I generally do not use chassis or turret retrofits because in PvE you rarely need them.  If you've gotten into a position where either of those is necessary, you've probably made a major mistake and you need to learn to avoid that rather than wasting a retrofit slot on a preventative measure.  Both are similar to the aim time retrofit in that they help you come to bear on target faster, but aim time is always useful (for literally every shot) whereas chassis and turret enhancements are only occasionally useful (once you're pointed in the right direction you no longer need them until you move again).  That said, if your play style is to get in close and brawl with the enemy, then they can be very useful.  So you have to keep in mind how you play when choosing retrofits.

First, you should probably be using Vincent Girard as your commander for the Challenger series, because he has mobility boosts that really help these unwieldy tanks.  He can also boost health by nearly 15% which is nice on any MBT but especially good on Challengers and Merkavas because they already have large health pools to start with.  Girard can put a Chally over 5K health.

Second, retrofits will depend on both how you plan and what game mode you're playing.  I can only comment on PvE because that's all I play.  PvE is all about doing as much damage as you can as quickly as you can, so retrofits are generally chosen with that in mind.  That's done with gun retrofits and vision retrofits (you can't shoot if you can't see).  In PvE, every vehicle should be using the Improved Gun Breach v2 retrofit which increases rate of fire by 12.5%.  It's the single best retrofit for PvE and short of some really weird corner cases I can't think of any vehicle that doesn't benefit from having it installed.  After that I look at the vehicle's accuracy and aim time.  I prefer accuracy to be below 0.10 and aim time to be below 2.0 seconds (preferably 1.5 seconds).  So if those need adjusting, I use the appropriate retrofits.  Then I'll slot in the visual range enhancements if I still have room.

I generally do not use chassis or turret retrofits because in PvE you rarely need them.  If you've gotten into a position where either of those is necessary, you've probably made a major mistake and you need to learn to avoid that rather than wasting a retrofit slot on a preventative measure.  Both are similar to the aim time retrofit in that they help you come to bear on target faster, but aim time is always useful (for literally every shot) whereas chassis and turret enhancements are only occasionally useful (once you're pointed in the right direction you no longer need them until you move again).  That said, if your play style is to get in close and brawl with the enemy, then they can be very useful.  So you have to keep in mind how you play when choosing retrofits.