While I would never want to say that Quiller is not the best secret agent ever, I also feel compelled to point out that, compared to Schrenk, he doesn't even seem to be the best agent in Moscow at the time of the Scorpion mission: that honor should go to Helmut Schrenk.
If we compare what we know about the two agents, Schrenk, in almost every way, is clearly the superior. Here are the points of comparison, and the agent to whom the "advantage" goes:
1. Ability to escape from opposition confinement. ADVANTAGE: Schrenk. Schrenk (note: throughout, I adopt the convention of referring to Shapiro as "Schrenk") managed to escape from *inside* Lubyanka, following torture. Quiller escapes from a van *on its way* to the Serbsky Institute, before any torture has occurred.
2. Resolve to "follow through" in executing the goal of a mission. ADVANTAGE: Schrenk. Schrenk has no problem blowing Q, Gorsky, and anybody else who gets in his way, to the KGB. Quiller places his entire mission in jeopardy because he cannot bring himself to kill Schrenk, as Croder ordered him to do. [Note: Schrenk, however, does appear to lose some of his resolve after facing Q, so that he is not able to pull the trigger himself in the apartment. However, this may simply have been a matter of convenience, Schrenk not wanting to have to get Q's body out of the apartment, relying only on himself, Misha, and the out-of-condition Ignatov.]
3. Ability to self-direct activity in the field. ADVANTAGE: Schrenk. Despite Q's oft-professed obsession about working alone, Scorpion is one mission where he pretty much makes a mess of things, until Croder arrives with a six-man support cell to take over the mission. Schrenk, on the other hand, nearly pulls off a classic assassination with a rag-tag team of ill-trained amateurs, and virtually nothing in the way of support from any agency.
4. Resistance to interrogation. ADVANTAGE: Schrenk. If Q gets a "9," Schrenk deserves a "27" or a "36." Despite having been physically brutalized in Lubyanka, Schrenk resisted interrogation by means of the extraordinarily creative strategy of moving scenarios down to the subconscious (Q never does that, does he?). Quiller, on the other hand, nearly comes apart at the seams ("I think in another twenty-four hours he would have got me blurting things out between hallucinations" [10:90]) over somebody whacking the table and a little sleep deprivation.
5. Moral compass. ADVANTAGE: Even. While Schrenk clearly has in mind the goal of removing what he sees as a threat both to Jews and non-Jews in Russia, one would find it difficult to disagree with Q's justification for stopping him, well-articulated by Croder: "The situation at this moment is that an attempt on Brezhnev's life might be made and might succeed. If it succeeds, the interests of Russian dissidents will suffer unimaginably in terms of reprisals, since some of the action group are bound to be caught. But if we even leaked a warning to Russian security the repercussions could be disastrous, not only for the Jewish dissidents but for East-West relations, even if no attempt were made at all." [17:153] Thus, we are presented with one of those moral dilemmas Hall is so good at creating, with pro- and antagonist each morally right in their own realm of experience.
6. Ability to learn from one's tradecraft mistakes. ADVANTAGE: Quiller. Schrenk's problem is that, ultimately, his operation falls apart because he cannot adjust his behavior, one presumes due to his obsessive focus on striking a blow against the oppressive Soviet regime. Rather than attempting to finesse Quiller by keeping him captive or blowing him to the KGB, he sends him out to be killed by a man extremely ill-suited to complete the task. Instead of laying low in the face of known penetration by the Bureau of his operation, he pushes ahead to the disastrous conclusion. Quiller, on the other hand, learns progressively more about Schrenk, until finally he is able to "read" what's in Schrenk's mind, permitting him to lead Schrenk to his destruction.
Perhaps the lesson from all of this is that, providing one can survive, it is mistakes and what one learns from them that ultimately prove to be one's most valuable tradecraft resources.