In my opinion, the only set of Pokemon that actually holds well on its own is the third generation. The second gen "completes" the first gen, while the 4th gen is sort of a reconciliation of the first three. The 5th gen is probably the second closest to being a "complete" set, but to me it seems like they purposely made it open for a sixth gen.
Are you kidding me? The 5th Gen is the most complete Generation we've had. 3rd Gen still builds on previous gens with Wynaut and Azurill, and besides that, the 3rd Gen wasn't as great a mix of Pokemon as the 5th Gen. Meanwhile, the 5th Gen is 100% separate from previous generations. And I fail to see how it "leaves itself open for a sixth gen." We have only a few standalones that really have room to grow, and a small amount of 2-stage Pokemon that can really be built off of (which is the same case for the 3rd Gen).
3rd Gen had a complete-ish feel, but 5th Gen takes the cake, hands-down.