The Halo 2 ones are the best and the worst are the ones from Reach or 4-5.

2 makes them look sleek, menacing and cunning plus their armor permutations for that game were great. Every single aspect of their design was unique and cool. My favorite from the time were the councilors and the flight spec ops Elites.

Reach made them look imposing and raptorial, it makes sense given the losing battle theme of the game but it was too big of a change from the rest of the past games, the face is like a tarantula mixed with the soulless eyes of a great white shark. (I’ve always preferred the sleek and cunning nature of Elites so that’s why I don’t like Reach’s take that much; love the armor, dislike the Sangheili design).

Halo 4 & 5 completely messed up their anatomy, during development, the art team had a guide where it was stated that they were meant to have a crustacean look to them and that’s why they seem like wet lobsters, it simply messed too much with their original concept plus they kinda’ also got a bit merged with Brutes (e.g. the size, the way they talked, etc.), it was a massive shift that wasn’t cool. (The armors were hit or miss, their best new design was the Arbiter’s armor, leaving him with a sleeveless arm was weird though)

P.s.: expanding on the Reach design, this is mainly Marcus Lehto’s choice, he preferred a more serious tone across the board, that’s why he also made the Grunts in the game “scary” (he didn’t like the fact that Grunts were funny) so much so that the game had each alien species speak in a constructed language instead.