Celesteela, Chansey, Clefable, Ferrothorn, Gastrodon, Skarmory, and Toxapex come to mind for clear walls. They're complete stops to certain Pokemon without a means of answering them. Generally, these Pokemon specialize in stopping certain types of attackers. For example, Chansey is the quintessential stop to most special attackers. It synergizes well with physical walls like Celesteela and Toxapex that can sponge physical attacks meant for Chansey. Usually their offensive presence isn't as overwhelming as some of the wrecking balls in the metagame, but often they can stall out enemies with status moves or just outright kill them in the case of Ferrothorn and Chansey. Status effect moves, Leech Seed, and a form of recovery are nearly omnipresent on sets for them, but they may find themselves getting run down by some of the overwhelmingly powerful attackers in the tier or stopped hard by a certain Pokemon. For example, Skarmory and Celesteela might completely stop attackers like Mega Pinsir, but once Magnezone hits the field they're dead without a Shed Shell. Chansey might be the best special wall one could ask for, but many attackers run Psyshock just to break the egg.
Pokemon like Buzzwole, Gliscor, Jirachi, Mega Latias, Rotom-W, Tangrowth, Mega Venusaur, and Zapdos are moreso tanks. They're fairly bulky and may sometimes even wall certain Pokemon cold, but they aren't as bulky as straight-up walls. Many of them do possess good offenses along with good bulk, as opposed to frail glass cannons like Greninja or pure stalling wall prowess like Chansey.
I didn't mention Landorus-T, Heatran, or the Tapus above, how come? Well, they're so versatile that they can run a myriad of sets. For example, a Landorus might run a suicide lead, or an attacker, or even a bulky set. I feel these are more so in a bit of a limbo where their role can easily be defined by what the player wants. Otherwise, they're closer to tanks.