Yanmega: Strictly worse than Togekiss. Much less bulk, as many weaknesses, worse resistances.
Vespiquen: Less tanky than Yanmega, glass cannon tier tankiness.
Gallade: Without double resistance to fighting, it's not even nearly as potent at stopping Machamp as Gardevoir. The best fighting move in the game currently that it could learn is Focus Blast, which is awful for defense and will never fire against an attacking Tyranitar before Gallade is dead.
Rotom: Glassier than Gengar. Even if people get confused about its type, it'll go down fast.
Spiritomb: Another extremely frail mon, glassier than Alakazam.
Garchomp: Double weak to ice, gets wrecked by whoever our new ice king ends up being. Keep your Garchomps for attacking, they're likely going to be amazing for that.
Togekiss: Actually the most potential gym defender of the entire generation barring legendaries. With double resistance to both fighting and dragon, it's a wall against the most popular attacker unlike we've ever seen. It's fairly tanky and even taking SE damage takes almost as much time to kill as Gardevoir does with neutral damage, which somewhat mitigates the fact that its easier to counter than Gardy.
Another bulky pokemon would be Hippowdon, which as a pure ground type doesn't have that many exploitable weaknesses and could for example be placed after Togekiss to force attackers to change from Raikou or SD Tyranitar.
EDIT: Gamepress has all the gen 4 stats in the pokemon list already where it's easy to check them (although the legendary stats are pre-nerf). You really should get used to checking the actual stats before you claim that something is tanky when it's in fact extremely glassy.