Arbitrage might be the easiest in a sense as you can see the returns that are available to you. The only thing is the margins can be tiny sometimes and you need to execute the bets quickly to maximise profits. Also, betting companies pick up on this rather quickly and they can ban you if they spot suspicious activity on your account.
Yes, that's a good point - the fact that you can quantify your edge (almost 'visualize' your profit) can help to make this profit strategy very transparent for the user.. which might make it 'easier' for many. But it has never been an option that has been interesting for me personally. I really like the 'romantic' idea of getting an edge using my better understanding of markets or sports.
I would like to suggest profit strategy
#4: Trading on betting exchangesThis is when you make a bet in the expectation that the price will move and allow you to make the opposite bet ("lay" bet) at a good price. These two opposite bets, if made at the correct prices, lock in a risk-free profit, regardless of end result. It is easiest to do this on a betting exchange like Betfair where you can back and lay. But you *can* also do it on a sportsbook, most easily in markets where there are only two results possible. But really you need a proper betting exchange.
Anyway, it is a bit like day trading on stock markets - you are betting on the market movement, not the underlying fundamental. Not easy though!
This is a good link to explain more, on Betfair's own website:
https://betting.betfair.com/betfair-announcements/betting-apps/exchange-trading-for-beginners-290116-710.html