Value has to consider price, and with that in mind I would say that the Tenacious is the best value in the Spyderco line. It is low priced but has a nice design, good features, good construction, good fit and finish, and real G10. If you want lower price than that, the Byrd knives are good but the one that I own does not have the fit and finish of my Delica.
Depending on your budget, the Delica, Endura, Stretch and Native are good. Their prices are higher so strictly speaking their value will be a bit less. In the same price range of these would be a used Manix 2. The Manix 2 is a very solid knife and I don't know why it is so undervalued compared to other Spyderco knives, but because of this I would put it at a high value. The new Manix 2 S110V is a high value knife if you really want a primo steel. It is a combination of lower priced handle with very good blade.
Now if your price range is a little higher, go ahead and get the PM2. I have a lot of Spyderco knives and I think it is one of the best that they make, price no object. If you hunt around and find one, it is definitely a good purchase. Just be careful not to buy a fake.
In reality Spyderco makes a lot of good knives so there are a lot of reasonable answers to the OP's question.