No. 2: Best fit
A player often must fill two important things: scheme fit and need.
There are no argument against Bosa in this situation.
Bosa projects perfectly as an EDGE for a base 4-3 defense, such as the 49ers employ, and the Niners have a big need for two upgrades.
Bosa would project as an instant starter at EDGE, partnering up with defensive tackle DeForest Buckner as a potent one-two pass-rushing punch along the defensive line.
The waters get a bit murkier for Williams, as far as fit is concerned.
Buckner has one half of the defensive tackle spot locked down for as long as he wants, while Solomon Thomas has been an improved player sliding inside (one of the reasons EDGE is a huge need in the first place).
Williams projects to be a monster as an upgrade over Thomas and starting next to Buckner, but the need may not necessarily be there for Williams. He would start, but wouldn’t fix the needs that the 49ers do have.
Allen projects as a better fit for a 3-4 defense as it pertains to linebacker, thankfully for the 49ers he does fill a need as an EDGE in a 4-3.
His versatility does allow him to play as linebacker in a 4-3, but it would be a poor use of his pass-rushing talent. While obvious passing downs can see him moved to the EDGE, quarterbacks can pass on just about any down.
Allen needs to learn to put his hand in the dirt more. While he has played as an EDGE/linebacker hybrid at Kentucky, a lot of his plays on the EDGE as shown in his numerous YouTube highlight videos have him standing up.
The little things can matter when you have so many talented players to go for.
Winner: Nick Bosa
Bosa is the only one of the three to have no concerns whatsoever about scheme or fit in the 49ers defense. Williams would be an upgrade but doesn’t fill a need, while Allen has to learn to play with his hand in the dirt to be a true EDGE, while being only a partial fit as a linebacker that would need to be blitzing a lot or moved to EDGE on passing downs to take advantage of his strengths as a prospect.
It’s pretty trivial to knock Allen for only needing to learn the little things, but that’s how close these three prospects are.