https://bluebook.audiogon.com/brands/thiel-audio
...is basically a list of (true) Thiel's until just shortly after the founders passing and subsequent sale of his company's name (his own surname). Later and current Thiel's
, with very different designs and made in a different factory are considered by most Thiel enthusiasts as; Thiel's in name only. To their credit, the current Thiel owners are still offering service for Thiel's legacy models.
It gets a little more complicated after that. For the most part as one goes numerically up; the first digit will typically suggest that the speakers tend to have gotten bigger (though not always) and more expensive during their own time of production . Numbers after the decimal points offer a chronological advancement within that series.
Whether an older higher series (1st digit) or a later, perhaps more recent (after decimal point (when available)) lower series is better, is open to debate. The evolution of a series could be modest or fairly
extreme. Even within their own line some prefer the earlier release to the latter ones. Usually as one goes up the series (1st digit) the speakers got bigger (again there are exceptions) and offered deeper bass response. There are however exceptions. The CS 5's and CS 3's series being especially notable in this regard.
As to what's better is open to debate. On an absolute performance basis alone, I think many Thiel enthusiasts might offer the CS 5i, CS 7.2 and CS 3.7's as Thiel's best offerings. Other Thiel's might offer better value or just be better suited for different rooms, budgets, circumstances, or just personal preference.
Though the Thiel's share a house sound (or perhaps share less of one?;-)) the power requirements vary enough between series and even generations within series, that it would behoove one to pay attention to the specific models under consideration.