Optimus:
Animated- A Future Fire Truck with a Axe. OHHH YEAAHH!!!
Prime- A without the flame version of the Movie one.
G2 LR- A look that later become RID Scourge.

Megatron:
DOTM- A Hobo with a Minion and a Shotgun.
BW- Why not having a T-Rex's Head for a hand. YESSSS
Animated- Swords and G1-Look for him.

 

G1 for both.

 

Armada for both. I dont know why, I just love the originallity of the designs. Also, I love the IDW redesigns of characters, and Op/Megs are no exceptions. I also love the animated designs for the two of them as well

 

Optimus:

IDW Ongoing, WFC, Prime in that order

Megatron:

IDW ongoing, Prime, Animated in that order

 

Optimus - RiD super mode
Megatron - Animated; his first transmetal body from BW is cool as well.

 

I don't think I can limit it to just one for each.  

Optimus Prime:

1.) Bayformers (specifically DotM)
2.) TFP
3.) IDW (like, the one that's in Revenge of the Decepticons)

Megatron:

1.) Cybertron (Batmobile from hell)
2.) WFC/FOC
3.) IDW Stealth Bomber

 

For Designs only and not character. Then for Megatron, I can't make up my mind between TFP and ROTF. And Optimus I can't make up my mind between G1, ROTF and TFP.

 

Optimus:
TFTM
TF: DOTM
FOC
WFC
G1
IDW
Prime

Megatron:
TF DOTM (Yes i liked it lol)
FOC
WFC
IDW
G1
Prime
TF:Animated Earth and Cybertron

 

I honestly don't have a favourite Optimus design. I have favourite ascects of tons of designs but I don't have one that i can say is my favourite. I you combined those you'd get my fav design.

But if I just HAD to choose one I'd go with G1.

Megatron on the other hand is hands down the Animated earth mode. Simply the best Megatron design we've got so far.

Special props to Prime Megatron but I still hate his shark teeth.

 

For megatron G1 because he turns into a pistol.

 

Optimus: For All Mankind IDW Comics.

Megatron: Animated

 

Optimus Prime:

1. Animated
2. Cybertron
3. Tie between RID and G2
4. IDW
5. Armada
6. Rescue Bots
7. Prime
8. G1
9. Movie
10. Energon

Megatron:

1. Animated
2. Cybertron
3. Beast Wars (All 3 forms)
4. Tie between DOTM and G2
5. IDW
6. Prime
7. G1
8. RID
9. Armada
10. Energon

 

This. Except Fatimus. I'd put the G2 design.

 

My favorite optimus prime designs are:
1 galaxy convoy
2 06 classics voyager
3 fire convoy

For megatron
1 g1
2 g2
3 cybertron (uniceon trilogy)

Those are the beat to me. Though as an honorable meantion, rid megateon being a multi former, is possibly the greatest evolution for megatron since megatron would want the most powerful body of all transformers.

 

Prime

1)G1:has aged well and is still iconic
2)DOTM
3)Cybertron

Then,also Prime and Animated are good.


Megatron

1)Animated:reminds a lot G1 counterpart and he's imposing.And,has a great leader toy.

 

Optimus: None really

Megatron: Prime, 1st movie, G!

 

The Bay movies gave us a great Optimus, so I'd give my vote to that just over the Cybertron/Galaxy Force design.

For Megatron it's G2 tank, I don't think anything comes close to that design. Animated Megs coming in a distant second.

 

Optimus: g1 it so classic and every prime but r.i.d is based off of this design in some ways.
Megatron: animated it a good reimagination of g1 without being a gun.

 

G1 for Optimus

And the Battlestar designs for Megatron

 

for both characters - cybertron/galaxy force (there g1 characters are icons in there own)..