Possessive and plural of "Series"

Solution 1:

All correct but the last one.

If you are talking about one series:

This series' value is unmatched

If you are talking about multiple series:

These series' values are unmatched.

If you are talking about multiple series valued together:

The value of these series' is unmatched.

Solution 2:

I think I agree with HorusKol. It's not wrong to say These series' value is unmatched, just awkward.

Think of similar examples with other nouns whose plurals are identical to their singular forms:

This sheep's fleece is white

These sheep's fleeces are black

These sheep's field is enormous

You may choose to construct a periphrasis to avoid confusion (especially if the line is to be spoken rather than read), but it's not incorrect to use a concise form from which the context makes the meaning plain.