Depends on what your looking for. Heres what I can think of off the top of my head:
Synergy wise:
Strange-
Pros:
Great Synergy with Viper and Strider.
Gets TAC palm loops, and EX TK hard tag loops on clean hits, which lead to meter positive kills.
One of vipers best assist, and Strider can get milage off of Strange’s assist well so you can have a versatile team.
Cons:
Strange can get mauled if you don’t know his neutral.
DHC is sub par
Match ups.
Zero-
Pros:
Decent assist with alpha, and beat assist especially in sougemu.
TAC infinite, and DHC loops.
You get the zero strider combo, what more could you ask for?
Cons:
No life?? Really theres no con besides sacrificing Vipers best assists, for possibly the best character in the game.
Rocket Raccoon-
Pros:
Good neutral with all assist.
You get a character who can do teleport Vajra in the neutral as well.
Good DHC with trap super hard tag/do what ever you want.
Cons:
This team can be reset heavy if viper dies and XF is spent. However they both have tools to do it.
Ammy-
Pros:
Vipers best Lock down assist hands down
TAC Infinite
Use’s strider effectively with stance cancels
Slow mix ups are very strong with all members on the team.
Strider can utilize cold star effectively
THC mix ups similar to doom/ammy with strider/ammy but MUCH harder to block
Cons:
This team can be reset heavy if viper dies and XF is spent. However they both have tools to do it.
Iron Man-
Pros:
Uni beam is very strong with all characters on the team.
Repulser is also a good assist if you want to switch it up
TAC infinite
Iron man can use Vajra effectively
Cons:
You have to play ironman, which isn’t a bad thing it’s just going to take some work in the neutral.
Match ups.
Akuma-
Pros:
Tatsu is a great neutral assist. It can help provide Viper with more durability making her seismo corner push even scarer and harder to deal with.
Can use strider effectively and vice versa.
Ground Bounce is still available after THC and you can double super from THC.
Cons:
Don’t really play Akuma so I couldn’t tell you, however it’s a very strong team.
Possible match up issues?
Nova-
Pros:
All assist are decent for viper as she is a versatile character.
TAC infinite
Can utilize strider effectively.
Cons:
Assist are not the best for either character.
DHC is decent.
M.O.D.O.K-
Pros:
All assist are very useful.
Ballon bomb is great for establishing space while dominating the ground with seismo in neutral.
Psionic blast is a good beam assist for both characters.
Barrier is good for seismo neutral control, its sort of like a mini focus attack but viper is free to use normals, and its comes out even if M.O.D.O.K gets hit during it.
TAC infinite
Good DHC that allows for follow ups after the super, ala install supers.
All members on the team can utilize all assist effectively
Cons:
Big hit box.
Weird neutral.
Skrull-
Pros:
Beta and Gamma assist are decent.
Beta has somme super armor and makes it a decent tool for both characters. It’s like a mini tatsu
Gamma is good for unblockables.
Can utilize strider effectively.
Decent DHC’s
Since viper doesn’t use ground bounces of wall bounces THC’s can be creative. Especially if you have an assist still available.
Cons:
Assist for neutral, while decent, are not the best.
DHC is not the best.
Dead Pool-
Pros:
Alpha and Gamma assist are decent during neutral.
Alpha is a low hitting assist that travels nearly full screen! Great for both characters.
Gamma is good for cutting of an angel of the screen strider would not hit.
Decent DHC’s.
Ground bounce and wall bounce are still available in combos from a TAC.
Can utilize Strider effectively.
Cons:
Assist are just decent and not great.
DHC’s are decent and not great.
Hawkeye-
Pros:
Alpha and Gamma assist are great for neutral for all members.
TAC’s leave you with ground bounce available.
Can utilize Strider effectively.
Cons:
Hawkeye doesn’t dominate the neutral enough to be in the second slot.
DHCs are decent and not great.
Match ups.
Chris-
Pros:
All assist are great.
Alpha is great for combo extensions and creating some scape is you can cover him with seismo/laser. If he hits during neutral thats a free combo from the hard knock down!
Beta is the best assist for horizontal coverage during a seismo press.
Gamma is more specialized, it can be used to cover yourself in the neutral a lot if your standing right in front of/on it and its a great combo extender.
Ground bounce and wall bounce still available during TAC.
Great DHC that can leave you meter positive if you have an assist available still.
All members can utilize his assist and Chris gets good milage off Vajra.
Cons:
IMO his only real con are his match ups, but strider can cover that issue up.
Thor-
Pros:
Decent assist.
Beta is good in the middle of frame traps for a nice party starter.
Gamma is REALLY good for seimso push! It nullifies low and medium projectils and on hit leads to a combo, and comes out and a nice angel!
Lots of health
Both characters can find a use for assist although they are better with Viper the with Strider.
Thor can utilize Vajra.
Decent DHC
TAC infinite
Conter projectile character.
Cons:
Slow.
Bad projectile game.
Assist are just decent and not great.
Im sure theres more I’m missing but thats just a few characters off the top of my head.