R2D2 and C3PO's long back-and-forth conversations? When did these take place?
How about the whole beginning of ANH and RotJ? Plus a fair chunk of the end of ESB.
They spend a fair amount of time bickering in AotC.
The reason why it's a back-and-forth is because R2 s clearly introducing IDEAS, which C-3PO is then RESPONDING to.
I.e. in AotC, R2 continually insists that they leave the ship to help Padme and Anakin. "For a mechanic you seem to do an excessive amount of THINKING."
In ESB, R2 is clearly upset at being shocked by the power terminal, and C-3PO RESPONDS to him: "Don't blame me. I'm an interpreter. How am I supposed to know the difference between a power outlet and a computer terminal?"
Nearly all the dialogue in the first quarter of ANH is between R2 and 3PO. R2 decides to go in a particular direction and DEFENDS HIS DECISION. "What makes you think there are settlements over there?...Don't get technical with me."
The second scene of RotJ is all about them. 3PO is in fact led by R2 in the situation, attempting to leave after a cursory effort to enter the palace but then following R2 inside, trying to convince him that it's not such a good idea.
This is all off the top of my head, you understand.
You don't even know what R2 is saying, for fuck's sake.
Heck, in a few cases inference tells us EXACTLY what R2 is saying. When 3PO says "Of course I've looked better" he's obviously responding to..."You've looked better." A translated readout of R2's statements is made available to Luke in some scenes, and they have a back and forth..."We're not going to regroup with the others. We're going to the Dagobah system...Yes, I'm sure it's perfectly safe for droids." Later Luke admits, "You thought coming here was a bad idea...I'm beginning to agree with you."
Come to think of it, in TPM Anakin has a readout of R2's statements, and the droid makes a recommendation...that he go back.
Several of R2's lines are translated into English for the reader of the RotS novel, including an ENTIRE conversation with 3PO in which R2 expresses concern about Anakin's actions and emotional state.
In fact, just as an exercise, I will now turn your tactics back upon you, to show you how ridiculous they are.
Good luck. There's a hell of a lot more evidence in R2's case. If battle droids showed half of this guy's initiative and intelligence, I wouldn't be questioning your assumption.
Prove to me that R2D2 is sapient. Just for the sake of argument, I now say it's unlikely, that his responses could have been nothing more than a low-end AI routine, and that people wouldn't waste resources building intelligent brains into astromech droids which are only supposed to serve a specific mechanical function and which are necessarily mass-produced.
R2-D2 is obviously not a low-end AI. Several examples are above. Also:
He takes initiative in AotC, arguing with 3PO about leaving the ship, doing so, shoving 3PO out a door in order to fly to a control console and manipulate the factory controls in order to save his master, Padme. This is without being asked, or even sure that Padme WOULD be in danger that he could help her with.
The RotS novel shows that Kenobi, who has no respect for battle droids whatsoever, is incredibly impressed by Anakin's R2 unit and considers it nonstandard. (Raising the question of this debate's relevance...we are trying to compare standard battle droids to a non-standard R2?)
In ANH, he disobeys Luke and strikes out in search of Kenobi alone. He also tells 3PO to "use the comlink," reminding him that he has turned it off.
In ESB, when cut off from Luke he searches for and finds some of Luke's allies. Perhaps he figured that that was his best chance of finding Luke again.
In RotJ, he attacks Ewoks due to percieved mistreatment.
Addressing the last part of your statement, astromech droids aren't built in the quintillions, and they need to be pretty smart in order to be useful fixing and flying a large number of ships. In addition, R2 is a decidedly non-standard example, having been heavily modified by the Royal Enginners on Naboo (and god knows who else).