I wouldn't go that far. It's not among the best of even the larger Hammer Frankenstein series. Still, it does have its charms, the biggest being Peter Cushing in the lead role. Brian likes him, too:
"The ultimate success of the Hammer series derives from Cushing's making the part all his own."Cushing is very good in the part. One hesitates for a moment about calling him the definitive screen Frankenstein only because it may be somewhat misleading, as Cushing didn't just play one Victor Frankenstein in that series--he played half a dozen.
It's fortunate that Terence Fisher directed most of those movies and not just because he was a competent director. Like most of the Hammer hands, he was a craftsman rather than an artist and while his lack of any significant cohesive personal vision for the series allowed for discontinuities galore, it also meant we got lots of different versions of Frankenstein out of it.
One--the one introduced in THE CURSE OF FRANKENSTEIN--is the relatively one-note mad scientist playing at being a god and in the end being punished for it. The next was introduced in the next film, THE REVENGE OF FRANKENSTEIN, and is almost the polar opposite, a compassionate (if, shall we say, sometimes overzealous) fellow whose work has a genuinely humanitarian aspect but who, in the end, is undone by the ignorance and bigotry of others. The succeeding films all offer, speaking broadly, either variations on one or the other or combinations of both.
I'm divided on which I like best. I find the REVENGE version a lot more interesting. It's certainly a more complex character and he's partly likable because he really is partly likable (as opposed to being likable for being so charmingly evil). The CURSE Frankenstein doesn't have a great deal of depth and I've never cared for the idea of a scientist being punished for traipsing into the domain of God--it has an inherently anti-science bite to it I find quite distasteful. On the other hand, Cushing is an absolute blast to watch when he's playing Just Plain Bad. In my favorite moment from CURSE, Paul, Frankenstein's teacher-turned-assistant-turned-critic, objects that what he's doing is inhuman. Victor retorts, "I'm not harming anybody. Just robbing a few graves." On a page, that doesn't look like much. The thing that makes it so great is Cushing's delivery--it's done offhandedly and with utter sincerity, thrown out while he continues what he was doing, as if he can't even imagine how anyone could possibly object to it. It never fails to make me laugh. The film featuring the most extreme example of this version of the character--FRANKENSTEIN MUST BE DESTROYED (1969)--is also widely regarded as one of the very best of the series.
This author has some reservations about that last. Fortunately, I don't have to pick a favorite. Cushing played them all and he was very good at every one of them. Even when some of the ideas embraced by the films went right off into the ozone (soul-transplants, anyone?), Cushing could still keep a straight face and sell them to an audience. Hammer's Frankenstein films, like all its series, are uneven in quality, but even in the least of them, Cushing is always spot-on. Only once was he replaced in the role and the resulting film sucked like space, which just underlines more strongly that, to the extent those films can be said to work, he was a very large part of what made it so.
 That shouldn't be read as a slam of Fisher--continuity within all of Hammers' horror series was tenuous, at best.
 The theme doesn't really feature in the original Mary Shelley novel either, where Frankenstein's sin wasn't creating life but in neglecting the creature he'd given life.
 Though not, it should be said, as uneven as most of the others.