"Basic Target Millennial" here, I've been to all of them in the area and my Target power rankings are:

1. Marlborough West - conveniently off 290 and near the mall, rarely crowded, well-stocked.
2. Westborough - Route 9 is a pain but if you know shortcuts it's not bad to drive to. Also well-stocked, but the customers are downright aggressively entitled.
3. Marlborough East - Lower in the rankings only because it's a supreme pain to get to unless you're already in that part of Marlborough (it's across town from where 290/495 lets you out, so you're driving 10-15 minutes off the highway). But it's a "newer" Target and a very pleasant shopping experience.
4. Millbury - The rest of the stores in this shopping center boost Millbury's rankings higher, but it's really just two Worcester Targets in a trenchcoat. It's either well stocked or looted, no in-between, and like others have said do NOT buy fresh food there. It also hasn't had the Target Remodel yet so the layout is cramped.
5. Leominster - This would be higher if you didn't have to take 190 to get here, but a 10/10 shopping experience. Quick curbside pickup, well-stocked, and a very large store.

I'm not as well-versed in Walmart shopping, but I always go to Leicester or Westborough, whichever I'm closest to when I need to go there. The Hudson Walmart is, and always has been, terrible. West Boylston is a throwaway Walmart in that it's fine but I don't know anyone who goes there as a first choice.