In reality, an ISFJ is much more likely to be confused with an INFP though. I suspect there are a few ISFJs in INFP clothing around here and on other MBTI boards. People mistake their nurturing and self-sacrificing with INFP idealistic driven compassion and emotional healing. Two very different things...
I would go with ISTJ based on your description. I confused an ISTJ once for INFP based solely on the fact that he had a very altruistic streak. He volunteered a lot, and generally took a strong interest in helping people. However, SJs are duty-fulfilling types, and often have traditional values that include caring for their community. Their motivation comes from a different place, and they tend to aid people in practical ways and by doing things for them. I think NFs love that side of them, and we tend to want to own and identify with what we love.
A few other confusing qualities:
ISTJs can seem crusty on the surface, but may seem friendly to strangers because it is appropriate.
INFPs can seem cranky because Fi doesn't care about what it "should" care about, but when Fi does care, an INFP can seem very warm and caring.
Sardonic sense of humor....INFPs will be more abstract though, and it will hit you randomly. ISTJs tend to be more observational and dry.
From what you're describing, my dad is more ISTJ. He doesn't seem driven by humanitarian needs, per se. He's more technical. The word "Crusty" has exactitude.
I can personally relate with the random, abstract, snide remarks. They usually occur when I see someone as being meticulous or irrational.
ENTJs and ESTPs are both extraverted utilitarian thinkers, so it's easy to confuse the two.
I've pointed out before how those two types can appear similar to each other, but a lot of ig'nant people on this board don't believe me.
It depends on the individual. Perhaps you know some ESTPs who can be confused with ENTJs, but other people on this site don't, so they naturally won't believe you. (Actually, I do. When I was still new to typology, I initially typed one guy as ESTP, but after I studied it into depth it became clear he was unmistakeably an ENTJ. The "cover" was similar, the "inner principles" definitely not. He wasn't after fun, he was after power. ) Judging from the functions, ESFPs could be more likely confused with ENTJs, and I am acquainted with one ENTJ girl whose lifestyle was very ESFP when she was a teenager, but saying that it's a rule would be a stretch, and even Forer effect.
You seem to imply that ISFJs are more likely to be confused with INFPs than ISTJs, well, in some cases that's true, in others - absolutely not. For example, my INFP father has very strong Si and Ti and is a workaholic, hence he could outwardly seem like an ISTJ, and my INFP brother has apparently taken the same route, taking up Te for Ti, which made him extremely hard to type at first, but the thought patterns always give people away. People with high Ne always primarily look for possibilities and always try to balance people who are too "convinced" about their own truths, because for them, no truth is definite. Even when they seem very convinced themselves, it's only because they've already explored all the other possibilities and finally decided than one of them agrees the most with their own principles, be it Fi or Ti. (But this is already off-topic...)
To the OP, I agree with the ISTJ typing (your father actually sounds like a very strong ISTJ). Fe is fairly easy to spot and nothing you described indicates it, so I'd exclude the ISFJ.
It depends on the individual. Perhaps you know some ESTPs who can be confused with ENTJs, but other people on this site don't, so they naturally won't believe you. (Actually, I do. When I was still new to typology, I initially typed one guy as ESTP, but after I studied it into depth it became clear he was unmistakeably an ENTJ. The "cover" was similar, the "inner principles" definitely not. He wasn't after fun, he was after power. )
Actually, I forgot to expound on my statement. From what I noticed, ENTJ men seem to be fairly easy to confuse with ESTP men, but not the other way around. I theorized that it is due to the fact that a lot of male ENTJs tend to adapt ESTP social tactics due to their seeming effectiveness... of course, some ENTJs on the board (*cough*Jaguar*cogh*) disagree.. but anyway, it was just my theory.
Judging from the functions, ESFPs could be more likely confused with ENTJs, and I am acquainted with one ENTJ girl whose lifestyle was very ESFP when she was a teenager, but saying that it's a rule would be a stretch, and even Forer effect.
ENTJ women, just as ENTJ men, prize control and competence. For a woman, a lot of her worth is derived from the fact of how many men desire her. I've seen many ENTJ women notice that their more "feminine" counterparts like ENFPs and ESFPs get more male attention, even though being not significantly (if it all) better looking than ENTJs themselves. So in turn, those ENTJ women would attempt to mimic their counterparts to obtain the same results.
Now why wouldn't other types of women use the same tactics as ENTJ women. Well the thing is, ENTJs, being NTs, tend to pay close attention to systems and patterns . And being extraverts, they value social competence greatly. So they see effective patterns of other socially "desirable" types and attempt to incorporate that into their own behavior.
I haven't seen ENTPs do the same thing that ENTJs seem to do. That's either because ENTPs already view themselves as sufficiently socially competent, or they do not care for implementing other type's abilities.
Listen to me, baby, you got to understand, you're old enough to learn the makings of a man.