too wide of a question to tell you.. but I can tell you my bf put one of his betas into a 125g. He's in there with some rather large south american cichlids some male rainbows, zebra loaches, angelicas botia, kuhli loaches, a couple different plecos, hmm... I can't recall what else. In larger tanks a lot of the "no-no's" are easier to safely try.
Agency 13 is correct: The question is too broad for a specific answer.
What you put in any tank with a Betta depends primarily on your knowledge of of various species and their habits. And your attention to detail. An experienced fishkeeper can get by with a lot of "don'ts" that an inexperienced fishkeeper cannot. And by "experienced" I mean someone familiar with more than just Bettas.
Are potential tankmates territorial? Are they aggressive or peaceful? Are they fin nippers? Are they slow shoalers or so busy they'd stress out any Betta? What size will they eventually become? Big enough to eat the Betta and other tankmates?
What parameters do the various species require? Can you meet them? Do the fish need a strong current? Do they eat plants?
What do you want your tank to be? A peaceful tank of small-to-medium fish? Or do you like the displays and territorial behavior of Cichlids? Do you eventually want shrimp or other inverts?
It's impossible to suggest tankmates without knowing your vision of an aquarium and the parameters in which the fish must live.
You could have a huge school of cory cats! I think cories are adorable. Tons of colorful snails, definitely; you'll want some tank cleaners. Shrimp are an obvious choice.
You could probably get away with guppies if it's well-planted enough. Or barbs, danios, tetras, that sort of thing, as long as you give them enough cover. But you probably don't want to stock as much of anything that bites fins as you normally would, because of the betta.
If you wanted, that would be a really good tank for a sorority. That's another option.
if it's well planted you could probably fit some fish that usually wouldn't go with the betta Like guppies danios snails but i would still avoid males because they will eventually get to each other and fight corys will be great for that tank who knows you might get an "oops" spawn from those cats