Atlético - Always seem to be pretty close to the title in any save I've had but I haven't been them personally but with players like Griezmann, Carrasco, Saul and their defence is magnificent so they would be a good team to try and topple Madrid and Barcelona.
Sevilla - Haven't really paid much attention to them, unlike Atlético they offer a greater challenge as they operate on a shoestring transfer budget (I'm guessing) but do have some talented players to work with.
Overall they would both offer great challenges Atlético are better suited to challenge immediately but Sevilla could be a terrific long term save which would give greater satisfaction if you won a title.
Sevilla budget is not necessarily shoestring from the 2nd season onwards and depends on success. Not as much as barca and rmadrid will have but enuff to be competitive and usurp them sooner rather than later