1 is obviously bad, ew.
2 is common and feels lackluster.
4 doesn't look nice. There's a reason chefs place an odd number of items on a dish.
5 while ideal, isn't realistic unless you have $$$ to throw out.
Left is Clodagh's Special Cheering Pants. They were from an old event but you can probably still find one somewhere.
If that option isn't available, you can try Pirate Greaves on the right. Personally, I think they look best as the cloth texture matches the tank top, unlike the ugly crinkles you get with Raider's and you have the layered metallic pieces on the knee which match the ones on the shoulder of the Santa jacket. If you're looking for something more plain, then Veteran or normal Santa should do.
You could try Terminus or K-9, but there's a bit of a clipping issue with the belt.
As for the boots, I find the Samurai ones in the depot to be a nice fit (seen with the Pirate Greaves). The way the laces are tied matches the cape tie and it's kinda the same texture as the vest and such. However, if that isn't an option for you, K-9 works fine I think.
Before, the choices might have had some impact on gameplay. But now that you can eventually max out every Transformation skill given enough time, I don't think it matters.
Grasp - Smashes send out damage dealing projectiles (it's not much. My ones deal like 900 damage each in Dullahan)
Divine Punishment - Increases Grasp projectiles to 4
Petrification - Petrifies 1 enemy hit by Grasp. Doesn't work on bosses
Active: Conviction - Become invincible while you send out projectiles and gain a shield afterwards. (Projectiles deal very little damage, but recover 250SP)
Discipline - Increases damage absorbed with Conviction by 100% (I'm unsure about shield health but it's around 10,000 or so)
Not sure about DKs, but their version of the projectiles slows enemies instead of petrifying and might have some other effect.
And, of course, their Active: Fissure. Doesn't do much damage either but "freezes" a boss for around 2 seconds and removes hit drags for anyone who hits the frozen boss. Think of Arisha's ultimate except it happens instantly. You should also be invincible for the duration and recover some SP.
Everything else is literally the same. Either choose the trans based off of those two actives or just the one you like the aesthetics of the most.