Blists Hill is a must. Lots of different things to see. The Tar Tunnel is also a must - but very short. I've never been too bothered about the pottery/tile museums etc.
Best deal is to buy their Passport to all the museums in the valley: it's valid for unlimited entry for a year into all their properties, but if you haven't seem them all in that time you can still visit.
we really enjoyed the clay pipe museum - were most surprised as it didn't sound too great, we'd left it till last as a "filler", but it's the only museum there the DDs still talk about. We were lucky htough in that they had a man there actualy making the pipes, adn he spent ages talking us through the different stages.