My Bernina isn't fussy at all about thread, so I use what I have, some of which is older than I am. When I need more, or a different color, I buy Gütermann Cotton or Coats Star Cotton. The only thread I've ever had a problem with is Sulky and about a dozen friends with different machine brands all had the same issue with it.
I agree with PaperPrincess. Try a small spool of several different brands to see what you and your machine like, what's easy to get locally, and what still leaves some funds in your wallet for fabric.