Depends on what you're looking for. Do you want to play competitively and gain ranks for the season rewards, or just have fun with it?

For just having fun with it I'll echo the recommendations of Brawl (probably Historic Brawl). For competitive, Explorer is probably the safest bet. It's a bit more competitive than Standard, but Standard is pretty high power nowadays anyway.

I play historic brawl. It can be slow to get into for someone new to Arena, but here are some ideas for budget decks: https://mtgazone.com/budget-historic-brawl/. Once you've been playing for a while, historic brawl is the cheapest, least wildcard intensive format. That's because it's singleton and non-rotating. I also like the randomness that 100-card singleton provides.

I should mention, however, that it's not as similar to edh as you might expect. For starters, it has a smaller card pool, 25 life instead of 40, and no commander damage rule. What makes a bigger difference, though, is that it's 1v1. Effective historic brawl decks usually pack more interaction, because you can't rely on the table to keep an opponent in check. Some people start historic brawl expecting to be able to use edh-style decks that build their position slowly or focus heavily on the commander without much protection for it. They are quickly disappointed, because those decks don't work nearly as well in historic brawl. Control is prevalent, aggro is fairly viable, and if you want your commander on the battlefield, be sure to run things like Slip Out the Back, Tamiyo's Safekeeping, etc.

I enjoy Explorer lately although the meta is very stale some days.

Otherwise Historic Brawl is fun. When I feel particularly casual I just skip opponents on there if I stare down the 6th Atraxa pile in a row.

Also, for me Explorer feels nicer than Historic lately due to the digital only mechanics taking hold in the meta. Just not my favourite style of play.