Are flat sole shoes best for cycling? ?

Depends on WHERE & HOW you plan on riding!  If you're just out for fun & a casual stroll on 2 wheels, there's nothing wrong with multi-purpose shoes.  However...the number one priority should be a stiff sole.  A stiff sole transfers more of your energy (power) directly into the pedals & throughout the drivetrain.  A soft sole will quickly become uncomfortable & quite possibly even painful.  

The word "best" is highly open to interpretation.  What's best for one person might be terrible to another.  The vast majority of serious cycling enthusiasts agree the "best" shoes (via clips in the sole) lock onto the pedals.  1) Your foot won't inadvertently slip off.  2) Power on the downstroke and once learned how...power on the upstroke too.  To un-clip, simply turn the heal out - away from the bike.  They're spring loaded & adjustable.  On two of my five bikes, I use dual sided multi-use pedals similar to those on this link...  https://www.amazon.com/dp/B07D1X1612   I can use stiff sole sneakers on casual rides & Shimano SPD compatible shoes with clips on more serious or longer rides.    

Repeating...the number one priority should be a stiff sole!  For both cycling & walking around with a flat sole shoe, I'd recommend these from Giro. https://www.giro.com/p/jacket-ii-mountain-bike-sho...  

Stop & think about this for maybe 30 seconds.  If flat sole shoes were best for cycling, why don't the worldwide pros and amateurs who race locally use them?  They don't.  They use shoes with clips or cleats that lock into the pedals.  

Road bike shoes have exposed cleats & are uncomfortable to walk round in.  Mountain bike shoes have the cleats recessed into the sole of the shoe & aren't so bad for walking short distances.  

As previously mentioned, you can wear flat sole shoes for casual cycling.  Just make sure they have a stiff sole.  The stiffer the better.  Flip-flops or sandals are a huge no-no.