The Armored Core series is best known for its in-depth customization. Here are some ACs that can achieve a "Great" Rating. I use these ACs, some maybe they can help you.

Note: These cost about 1Mil each, so its best to save up.

AC: 1
Default Name: Patriarch
Default Color: Base, Detail and Joint color: Blue 200, Green 100. Optional and Aid color- White.
(No Camoflague)
Parts:
Head- MHD-MM/007
Core- MCM-MX/002
Arms- MAH-RE/GG
Legs- CLH-XV-MSGR
Booster- CBT-FLEET
FCS- VREX-F/ND-B
Generator- KGP-Z5V
Radiator- RGI-KD99
Inisde- MWI-DD/10
Extension- CSS-IA-645
Back Units- MWC-LQ/15(L)
Arm Unit R- MWG-MG/1000
Arm Unit L- MLB-MOONLIGHT
Optional Parts:
OP-S-SCR
OP-E/SCR
OP-S/STAB
OP-L-AXL
OP-LFCS++
OP-L/TRN
Cost: 1,073,400
Grade: Great

This is a Heavy AC, specializing in Defense AND Offense. This is my Personal Core.

AC: 2
Default Name: Poltergeist
Default Color- Head and Left Arm- Black. Legs and Right Arm- 75 Red (For all). Core- Just mix them up.
(No Camoflague)
Parts:
Head- MHD-MM/007
Core- CCL-01-NER
Arms- KAW-SAMURAI2
Legs- MLL-MX/EDGE
Booster- CBT-01-UN4
FCS- VREX-F/ND-B
Generator- KGP-Z5V
Radiator- RGI-KD99
Inisde- None
Extension- CSS-IA-645
Back Units- MWC-LQ/15(x2)
Arm Unit R- None
Arm Unit L- None
Optional Parts:
OP-S-SCR
OP-E/SCR
OP-S/STAB
OP-L/TRN
OP-E-LAP
Cost: 1,104,600
Grade: Great

This is great for those mobility lovers. It excels in its speed, but lacks defense. The SAMURAI2 arms take off a lot of weight.

AC: 3
Default Name: Catalyst
Default Color- Grey(100 Blue, Red, Green) for everything.
(No Camoflague)
Parts:
Head- MHD-MM/007
Core- COM-OO-STO
Arms- CAM-14-DUSK
Legs- MLM-55/ORC
Booster- CBT-FLEET
FCS- VREX-F/ND-B
Generator- KGP-Z5V
Radiator- RGI-KD99
Inisde- MWI-DD/10
Extension- CSS-IA-645
Back Units- MWC-LQ/15(R)
Arm Unit R- MWG-RF/300
Arm Unit L- MLB-HALBERD
Optional Parts:
OP-S-SCR
OP-E/SCR
OP-S/STAB
OP-L-AXL
OP-LFCS++
Cost: 997,300
Grade: Great

This AC is very balanced, both quick and tough, it tackles the missions that require both skill and strength.

Note- You can change any part, or name, or color. These are just suggestions.