Are Cylons sentient?
 

I'm thinking no.

Sure, the original reptilian Cylons were but I think the race of robots that succeeded them aren't .

They're just individual robots pre-programmed to do individual tasks and part of a greater whole. While the whole is capable of taking over the galaxy bit by bit, an individual Cylon is pretty much worthless. A leader gives orders from information processed by others in the same series that has recieved information from drones and centurions in the field.

The only way you're going to stop them carrying out their programme is to turn them all off.

This theory is disproved by Cy in 'The Return of Starbuck' though. He could make his own decisions, including choosing to save human lives rather than kill them. This and his actions meant he had considered and chosen to switch loyalties. This is not something a toaster could do.

To build on yout theory :
Perhaps it was because he was a group leader and was given a higher level of awareness so he could make command decisions ? Certainly the IL series robots seemed to be sentient, so perhaps there were different levels of awareness/sentience/programming ?

I'm totally discounting Galactica 1980 even though I like it. I'm talking TOS, which many fans believe is canon.

Galactica 1980 is not canon it is a failed children’s show abomination.

Yes. Absolutely they are.
Wether you believe that they are 'mostly' or completely artificial, they certainly are sentient and conscious.
It could possibly be argued that the 'drone' centurion class, perorming the most menial tasks do not retain enough consciousness to be classed as sentient, but I'd argue that as well.
Certainly the Centurion class, Command class, Civilian, Worker, IL, and Leader are all IMO fully sentient and conscious.
The 'hive like' aspects of their society don't make them automatons.
There's no real evidence to support a belief that any of the interconnectivity of their society involves 'direct control' apart from that over the 'drone' class.
If you believe that Centurions are 'demoted' to drone class, then that has involve some kind of failure on their part to warrant the penalty of having their free-will removed. A 'dumb machine' does not fail, it simply performs its function to the best of its ability.
Vulpa is banished to Arcta for challenging the Leader in some way. Lucifer lies, so does Spectre. We see Centurions showing adaptive reasoning, questioning orders, 'sensing' things. Cylons are devious, self serving, in fact displaying everything from cowardice to irony and sarcasm.
Yes, they are sentient, just mostly or fully artificial.

Your absolutely right! And there are other examples of it.
Its not only Cy that makes that 'sort' of decision.
The Centurion that reports to Baltar during the ground invasion of Caprica in Saga chose not to impliment his standing orders from The Imperious Leader, when he kept the human captives alive, believing that the information they offered in trade was worth countermanding his standing orders.
Baltar needed to remind him of the standing orders and tell him to carry out his duty.
They are no different from any other soldiers. Yes there are group leaders, squad leaders, flight leaders, 'pilot leaders' in the crew of raiders, and 'raider leaders' in squadrons of raiders. Their command structure is no different from any other armed forces. In the high level 'Command' and above, there is certainly an element of 'upgrading' to give them expanded reasoning and tactical cognitive ability, but in the same way that some people are not cut out to be generals, but make good 'privates', Cylons are no different, and they are promoted/upgraded because of the ability and aptitude they show.

But most of them accept 'The Retrun of Starbuck' episode As gospel!