Congrats on your rescue sheltie!
Sit, down, stay and come are the usual basics. Once you have him performing them reliably at home, move to different environments (e.g. on a walk, at the park, at a mall, etc.) to make sure he will reliably perform them elsewhere.
Personally, recall (come) is the most important one for me to perfect. If there is every an emergency, I want my dog to come immediately when called no matter what else is going on. For this reason, I only give him positive experiences when he comes (e.g. treats, praise, play); I never, for example, call him to come for nail trimming, something which he hates.
Loose leash walking is another basic training area.