Auctions are definitely the best value, but they're also the most uncertain, you could get the same players as negotiations for just 1 token at a fraction of their price, or you might have to go to over 100 rounds and pay several times their market value.

You can get pretty good players on negotiations, but it depends on how greedy the manager is.

Scouts are the most certain ones, you're sure to pay 50 tokens and some cash, so pick good players, higher value and quality, and lower age.

For me, I only go for auctions, I need about 21 players a season on a budget of 35 tokens a season, and I always get them too. I am very selective with my bids.