Shooting
From Hattrick
This is a unique training type because it trains 2 skills: the scoring skill for all outfielders that have played and set pieces for all players, even those that haven't played. Only 1% of team owners train this. Note that scoring training also trains the scoring skil, for fewer people but at a faster rate, and Set pieces training also trains set pieces, but much faster.
Pros
You can train 20 players in scoring and 49 in Set pieces.
Cons
You have to forget Cup.