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.