With the wide selection of watch case styles available, one particular style seems to be the least favored of all in watch collecting circles, and that would be square, tank, and / or Tonneau-style cases and crystals.
It seems that the only watch, in the aforementioned styles, that has avoided mass hatred among collectors, is the Tag Heuer, Monaco; though there are surely a few others, but the Tag just stands-out to me. And for me personally, it's guilty as charged, as through my nearly 3 decades of watch collecting, I can count the number of square, tank, and tonneau case styles that I've owned, nearly using only one hand.
So what's the rub? I've felt that every square or tonneau-shaped case I've owned previously, has lived up to my quality expectation, but those watches never seem to have much longevity within my collection. For these case styles, the issue could be two-fold. Due to the design, most square cases will usually be engineered to smaller case diameters, to give the watch a more wearable profile on the wrist. Using larger dimensions on square timepieces, could leave the timepiece looking more like a brick on the wrist, while case diameters south of 40mm may leave this style of timepiece looking dainty on the wrist. The other issue, and most likely the biggest factor, square, tonneau, tank, and barrel shaped cases are typically reserved for more dressier, elegant looks; so the style doesn't lend itself to the wearable flexibility of a timepiece such as a dress diver. Limited wearing opportunities will always relegate a timepiece to the watchbox, limiting quality wrist-time.
As with all things style, it always comes down to individual style preference. But without question, square, tank, tonneau, and barrel cases haven't felt the universal love within the watch collecting community, enjoyed by round and cushioned case-styles, made famous by dive and pilot watches. And for many watch collectors, with regards to square and rectangle cases and crystals, its tanks, but no tanks.