In the handheld games (at least gen IV and prior, I haven't made my way through the later games yet.) there's a setting that asks you whether you'd like to go with the default or change to set mode.
The default, switch mode, is that when you knock out an opponent's Pokemon the game tells you what Pokemon your opponent is going to send out and gives you the opportunity to switch. Notably, the opposing trainers do not have this option.
Set mode simply means that your opponent will simply send out their next Pokemon after one faints. This is the default for online battling, and is what is used (to the best of my knowledge, which is admittedly not much) for competitive Pokemon.
I'll go with some updates/rehashing. Assumption is that I fail to catch anything before the list becomes relevant (and omitting anything present in the current encounter). No particular order, though Beldum/Phantasea/Naedire is at the top. But I'm honestly easy to please. EDIT: If I were to pick additional Pokemon from later gens this list gets a lot longer. Like I said, easy to please.
1. Beldum line
2. Phantasea
3. Naedire
4. Magnimite line
5. Eevee
6. Rotom
7. Mankey Line (Browsed Gen IX and Annihilape is really cool)
8. Bagon line
9. Electrike line
10. Slowbro Line
11. Feebas Line
12. Shuckle
13. Lucario Line
14. Spiritomb
15. Any tanky Pokemon with a type combination that includes two of (Steel, Psychic, Electric, Grass, Ghost)
16. A bunch of Pokemon from gen V+, with Mudsdale topping the list.
EDIT: After reading through the last IC thread, the Pokemon that taskmaster had at the shrine of light were both really cool, and could be put up there with Beldum. Phantasea and Naedire.