For pure wtf-ness: Thundercracker For a decently-proportioned cartoon-style representation: Bumblebee For giving the UK guys we missed out on first time around: Devastator, Shockwave, Blaster
Any of the ones with the big base/vehicle modes: Prime, Megatron, Gutcruncher, Wheeljack or Sprocket. They remind me of Joe vehicles in that your stand-alone guys can ride shotgun and take out the enemy while your main guy drives.