It depends on doctrine - is this an escort or a standalone combat ship?
For an escort, you want speed, light weaponry(anti-fighter, AMS, etc.), fighter/dropship capacity, and scouty things like NCSS. Armour is a secondary issue, and big guns should probably be avoided. The only capital-scale weapons I'd use would probably be NL45s and Barracudas, or sub-capital stuff if it's available. Maybe White Sharks if the doctrine is to degrade enemy capital ships with missile attacks before the big boys engage. NLs would be the bulk of it, because they're also good for fire support.
Class/Model/Name: Escort Destroyer
Tech: Inner Sphere
Ship Cost: $12,546,925,680.00
Magazine Cost: $10,880,000.00
BV2: 32,000
Mass: 200,000
K-F Drive System: Compact with L-F Battery
Power Plant: Maneuvering Drive
Safe Thrust: 5
Maximum Thrust: 8
Armor Type: Lamellor Ferro-Carbide
Armament:
32 AMS (IS)
32 LRM 20+Art4 (IS)
48 Laser Large ER (IS)
16 Naval Laser 45
16 Capital Launcher Barracuda
Class/Model/Name: Escort Destroyer
Mass: 200,000
Equipment: Mass
Drive: 60,000
Thrust
Safe: 5
Maximum: 8
Controls: 500
K-F Hyperdrive: Compact with L-F Battery (6 Integrity) 92,500
Jump Sail: (3 Integrity) 40
Structural Integrity: 50 10,000
Total Heat Sinks: 391(782) Double 0
Fuel & Fuel Pumps: 5000 points 1,020
Fire Control Computers: 0
Armor: 240 pts Lamellor Ferro-Carbide 200
Fore: 35
Fore-Left/Right: 45/45
Aft-Left/Right: 45/45
Aft: 25
Dropship Capacity: 2 2,000
Grav Decks:
Small: 1 50
Medium: 0 0
Large: 0
Escape Pods: 15 105
Life Boats: 15 105
Crew And Passengers:
25 Officers in 1st Class Quarters 250
60 Crew in 2nd Class Quarters 420
58 Gunners and Others in 2nd Class Quarters 406
72 Bay Personnel 0
# Weapons Loc Heat Damage Range Mass
4 AMS (IS) Nose 4 12 (1.2-C) PDS 2
4 AMS (IS) RBS 4 12 (1.2-C) PDS 2
4 AMS (IS) Aft 4 12 (1.2-C) PDS 2
4 AMS (IS) LBS 4 12 (1.2-C) PDS 2
4 AMS (IS) FR 4 12 (1.2-C) PDS 2
4 AMS (IS) FL 4 12 (1.2-C) PDS 2
4 AMS (IS) AR 4 12 (1.2-C) PDS 2
4 AMS (IS) AL 4 12 (1.2-C) PDS 2
4 LRM 20+Art4 (IS) Nose 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) Aft 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) RBS 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) LBS 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) FR 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) FL 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) AR 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) AL 24 64 (6.4-C) Long 44
6 Laser Large ER (IS) Nose 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) RBS 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) Aft 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) LBS 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) FR 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) FL 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) AR 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) AL 72 48 (4.8-C) Long 30
4 Naval Laser 45 Nose 280 180 (18-C) Extreme-C 3,600
4 Naval Laser 45 RBS 280 180 (18-C) Extreme-C 3,600
4 Naval Laser 45 LBS 280 180 (18-C) Extreme-C 3,600
4 Naval Laser 45 Aft 280 180 (18-C) Extreme-C 3,600
4 Capital Launcher Barracuda FL 40 80 (8-C) Extreme-C 360
4 Capital Launcher Barracuda FR 40 80 (8-C) Extreme-C 360
4 Capital Launcher Barracuda AL 40 80 (8-C) Extreme-C 360
4 Capital Launcher Barracuda AR 40 80 (8-C) Extreme-C 360
Ammo Rounds Mass
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
AMS (IS) Ammo 144 12.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
LRM 20+Art4 (IS) Ammo 120 20.00
Capital Launcher Barracuda Ammo 40 1,200.00
Capital Launcher Barracuda Ammo 40 1,200.00
Capital Launcher Barracuda Ammo 40 1,200.00
Capital Launcher Barracuda Ammo 40 1,200.00
Number Equipment and Bays Mass
5,000 Cargo, Standard 5,000
36 Bay Fighter 5,400
1 NCSS Large 500
For a standalone light combat ship, speed is still probably key to evade enemy battleships, but you want to up the armour a bit more, and probably pack a small number of hard-hitting capital weapons. Something like a quad HNPPC mount or a few NAC/40s in the nose - nose weapons don't need to be carried on each side(so you save weight), it's heavy enough to seriously threaten a battleship (especially in a high-speed closing engagement), and the issue of lining up with your target is less of a concern if you're zippy. Other angles probably carry missiles, because those can fire off-arc and thicken up the attacks, while not leaving it naked from other sides.
Class/Model/Name: Combat Destroyer
Tech: Inner Sphere
Ship Cost: $12,657,856,800.00
Magazine Cost: $12,480,000.00
BV2: 43,630
Mass: 200,000
K-F Drive System: Compact with L-F Battery
Power Plant: Maneuvering Drive
Safe Thrust: 5
Maximum Thrust: 8
Armor Type: Lamellor Ferro-Carbide
Armament:
32 AMS (IS)
32 LRM 20+Art4 (IS)
48 Laser Large ER (IS)
4 Naval PPC Heavy
12 Capital Launcher AR-10
Class/Model/Name: Combat Destroyer
Mass: 200,000
Equipment: Mass
Drive: 60,000
Thrust
Safe: 5
Maximum: 8
Controls: 500
K-F Hyperdrive: Compact with L-F Battery (6 Integrity) 92,500
Jump Sail: (3 Integrity) 40
Structural Integrity: 100 20,000
Total Heat Sinks: 691(1382) Double 300
Fuel & Fuel Pumps: 5000 points 1,020
Fire Control Computers: 0
Armor: 480 pts Lamellor Ferro-Carbide 400
Fore: 90
Fore-Left/Right: 65/65
Aft-Left/Right: 65/65
Aft: 50
Dropship Capacity: 2 2,000
Grav Decks:
Small: 1 50
Medium: 0 0
Large: 0
Escape Pods: 15 105
Life Boats: 10 70
Crew And Passengers:
22 Officers in 1st Class Quarters 220
63 Crew in 2nd Class Quarters 441
42 Gunners and Others in 2nd Class Quarters 294
# Weapons Loc Heat Damage Range Mass
4 AMS (IS) Nose 4 12 (1.2-C) PDS 2
4 AMS (IS) RBS 4 12 (1.2-C) PDS 2
4 AMS (IS) Aft 4 12 (1.2-C) PDS 2
4 AMS (IS) LBS 4 12 (1.2-C) PDS 2
4 AMS (IS) FR 4 12 (1.2-C) PDS 2
4 AMS (IS) FL 4 12 (1.2-C) PDS 2
4 AMS (IS) AR 4 12 (1.2-C) PDS 2
4 AMS (IS) AL 4 12 (1.2-C) PDS 2
4 LRM 20+Art4 (IS) Nose 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) Aft 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) RBS 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) LBS 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) FR 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) FL 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) AR 24 64 (6.4-C) Long 44
4 LRM 20+Art4 (IS) AL 24 64 (6.4-C) Long 44
6 Laser Large ER (IS) Nose 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) RBS 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) Aft 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) LBS 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) FR 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) FL 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) AR 72 48 (4.8-C) Long 30
6 Laser Large ER (IS) AL 72 48 (4.8-C) Long 30
4 Naval PPC Heavy Nose 900 600 (60-C) Extreme-C 12,000
6 Capital Launcher AR-10 Extreme-C 1,500
6 Capital Launcher AR-10 Extreme-C 1,500
Ammo Rounds Mass
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
AMS (IS) Ammo 240 20.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
LRM 20+Art4 (IS) Ammo 144 24.00
Capital Launcher Barracuda Ammo 60 1,800.00
Capital Launcher Barracuda Ammo 60 1,800.00
Number Equipment and Bays Mass
2,000 Cargo, Standard 2,000
1 NCSS Large 500
Note: Yes, I used AR-10s with only one ammo type. These can be re-equipped for different missions, while the dedicated Barracudas on the escort model can't be.