Fedureeer is the best overall. Agassi was good but not as good. Federer got the half-volley of the gods especially when on the move. Davydenko (in shape) was fantastic too. Key for him was the very quick footwork.
Agassi was maybe the best at early RoS. His BH return when stepping in was legendary.
Agassi. Before Agassi and modern strings shots weren't coming as hard & fast from the baseline, so it's very hard to compare, but in their day Connors & McEnroe both took the ball very early. Honourable mentions to Federer, Nalbandian, Davydenko. Djoker isn't bad either, though not in their league.
Davydenko --> Agassi (by a little fraction but Davy's a little faster)
In fact it is:
Agassi better than Davydenko in defensive half-voleys from the baseline. (Federer is better than Davy in defensive half-voleys)
Davydenko better than Agassi (so it means than everyone) in aggressive half-voleys from everywhere around the baseline BUT he can do them 3-4 metres inside the court too! and they'll still be aggressive. JUST LOOK some of his matches and you'll know it
I hear Agassi. Truth is, I didnt use to watch tennis in the 90s, have only watched highlights of some great matches. But what I can say is that even at 30, the Agassi I saw could take the ball on the rise like no one can - at least in today's game.
I definitely have to give the nod to Federer in terms of defensive half- volleys. How many times has he hit winners off of half volleys at the baseline? I see it so often. Even as recent as the AO, he was hitting half volley winners against Del Potro's super forehand shots! (especially with his BHs).
Overall, I would say Agassi. Honorable mentions to Nalbandian and Davydenko.