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Stash building? Cascade 220 is a wonderful yarn! I've never seen a Cascade 220 sweater that didn't look professional and beautiful. Cascade Superwash is also great.
Noro Silk Garden and Noro Kureyon...both great if you love variegated colorways. Noro Cash Iroha and Noro Iro: fantastic!
Tweeds are a great staple IMO. I adore Berrocco Blackstone Tweed and Rowan Silky Tweed are great stash builders.
Elsebeth Lavold Silky Wool is fantastic. It's been a best seller for years. It wears well and feels great. Warm but never too hot.
Another fav of mine is Black Water Abbey worsted. It's not a softie yarn, but after proper blocking I can comfortably wear it right next to my skin. It does wonderful cabled sweaters!
Yarn choices are so personal to each individual. For sweater knitting, spend as much as you can comfortably afford for the yarn. A quality yarn is worth it, especially for sweaters.
Cascade brand is noted for being high quality and affordable at the same time.