I enjoyed Halo 3's Covenant a lot. While they weren't as diverse as Halo 2 or Reach, Bungie designed them very well and the three most common species have a nice synergy.
The Brutes were fleshed out to fill a variety of roles, and they were no longer damage sponges that could kill you in one hit (except Hammer Chieftains). They were dangerous and relentless, but they were slower and had weaker armour than Elites. They could give orders to their underlings, showing off their radically different style of leadership. These orders tested how well you can react to your enemies and how well you know your surroundings. If you were hiding behind cover, enemies could come investigate or throw grenades at where they last saw you. Brutes also have hilarious lines, and their gruff snarling voices make those lines even funnier. Basically, they posed a challenge, but it wasn't an insurmountable challenge. You have a variety of options when fighting them on Legendary, unlike Halo 2 where you needed certain weapons at all times or dumb luck to proceed. Obviously some weapons are much better for the job than others, but none were totally useless like using a Plasma Pistol against a Halo 2 Brute. You can use automatic weapons, precision weapons, power weapons, or punch their lights out if you know how to kite them.
Grunts didn't really change much from previous games. They have no headgear and their heads stand out, but now they have a chance of going kamikaze when their leaders die. They also throw grenades on command, which makes them very dangerous in numbers. They're still cannon fodder, but they have a little more firepower when their leaders are alive.
Jackals retained their role as scouts and marksmen, but they're not nearly as infamous as their Halo 2 counterparts. They got headgear which tells you when they're watching you and where to shoot to take them down. Legendary snipers still kill in one hit, but are not nearly as numerous, don't spawn halfway through a fight, and take longer to aim and shoot. They're much more agile otherwise, as they will dodge explosives with ease.
Halo 3's Covenant didn't rely on gimmicks or cheap tricks like one-hit kills to challenge the player. They tested your ability to think and react. You had to think about which enemy to take out first, how to take them out, and from where, all while adapting to changes in enemy tactics, their movements, and your surroundings.