Possibly James Garner?
Laird at Swansea (not at Bournemouth though).
Henderson at Sheffield United.
But I take your point, not many have been really successful.
In the last couple of years alone you'd have to say Garner's loans to Forest, Laird's loans to Swansea, Dishon Bernard's loan to Salford, Chong's loan to Birmingham, Williams' loan to Norwich, Dalot's loan to Milan have all been pretty successful. Some other perfectly decent ones like Levitt and Mengi's as well. Just because they don't come back and instantly storm into our first team doesn't mean they didn't benefit from the loans. End of the day, not many players are going to storm into our first team, it's just how it is.
This January was not particularly brilliant though. I'm thinking of us working on a deal to send Laird to a club that aren't serious about giving him a chance instead of the one where he was starting every game, while seemingly not paying any attention to the situation of our £9m Uruguayan winger who can't get a kick at a dogshit Spanish team.