Which are best and why? I prefer dedicated uppers BTW.

Which have 25rd or greater capacities? DPMS, Ciener, anyone else?

Do any have a last shot hold open?

Are there some systems that are more reliable than others?

Which work with full auto? Ciener claims to be the only one.

At this point I am looking at either a Ciener or a DPMS, unless I can find something better. Of the DPMS or the ciener, which do you think is better?

Tsh77769

Which are best and why? I prefer dedicated uppers BTW.

Best is probably the Compass Lake upper.  Not the best value though, and I think only available in Service Rifle configurations.  DPMS is probably your best value right now, or the CZ if you're OK with a side charging handle.

Which have 25rd or greater capacities? DPMS, Ciener, anyone else?

Only Ciener as far as i know.  DPMS has promised hi-caps for a while, but they're not out yet.  Supposedly you can use the Ciner mags with the CZ upper.

Do any have a last shot hold open?

Maybe the Air Force M-261, but I've heard it's finnicky.  I think you can manually lock the Army M-261 open, but it won't lock on last shot.  Ciener does not and as far as I know the DPMS does not.

Are there some systems that are more reliable than others?

I think the kits are all about equal once they're tuned up.  Compass Lake thinks the M-261 is most reliable.  

Which work with full auto? Ciener claims to be the only one.

Ciener is the only one I know of.  

Of the DPMS or the ciener, which do you think is better?

If you don't need hi-caps I'd say they're equal.  DPMS is cheaper, especially for a dedicated upper.