Landorus has been surprisingly good for me, it's base 101 Speed is extremely helpful, picking off Scarved Palkia and Salamence, +1 Rayquaza among others. Although it seems outclassed by Scarf Garchomp with its base 102 Speed, threatening Dragon typing, and superior bulk, Landorus does have some advantages over Chomp: its access to U-turn (and a fairly strong one at that) is also a great way to keep momentum going for you. (T-)Spikes immunity is also a blessing in a metagame where spikestacking is so popular. It also gets a more powerful Earthquake in a sandstorm, which can really help in some situations.
On a related note, I've also tested Victini, but I'm not nearly as satisfied with it as I am with Landorus. Maybe it's just a question of the team I'm using or what but yeah. V-Create in sun is pretty darn beastly though.