If I am unaccompanied overseas and my spouse is stateside, what Tricare plan would I go under?

Not sure the exact nature of your question - if you are asking which plan would YOU go under, then if you're active duty, it's simple - TRICARE Prime. As an active duty member, you will always be TRICARE Prime. It may have different implementations of it (Prime Remote, Prime Overseas, etc.) depending on your location, but it will always be TRICARE Prime.

If by "what TRICARE plan would I go under" you mean "what plan is best for my family", then the option of "which plan" is completely subjective.

I could copy & paste all the information on the TRICARE site about the different plans*, but in a nutshell, TRICARE Prime is like an HMO and you will see your provider at a MTF. TRICARE Select is a PPO and you go to whoever you want as long as they are a TRICARE-authorized provider. Prime is cheaper with out of pocket expenses, but Select is more flexible.

Go to the "Compare Plans"* page and punch in the different plans and follow links for more details.
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* https://www.tricare.mil/Plans/HealthPlans
* https://www.tricare.mil/Plans/ComparePlans

That depends. Where is your spouse residing Stateside? I had to do Tricare Remote while I was in Honduras because my wife was more than 2 hours away from a military installation. Call Tricare and ask. [login to see]

Did your spouse remain in the area prior to your PCS? If so, she would remain under the plan she had prior to the PCS.
If she relocated to another area, she needs to contact Tricare to see if Prime is offered in the new area, if not, Select would be the option she needs to have.
Example: You were stationed at Fort Campbell, KY, her medical care was done at BACH, she remains in the Fort Campbell area, nothing changes.
If she goes home to say, Idaho, no Prime coverage, then she needs to get with Tricare to enroll in Select to have more choices for medical coverage.

You, as the military member do not change your plan.