Range time? Whatever you can get, regardless of weight.
Serious time?
There are about a half-dozen rounds at the top of the list.
110gr Tac-Tx (Many people have Barnes...as do it. But next time I will go with Wilson Combat's Tac-tx....it is loaded hotter)
110gr Hornady Black Vmax (what I keep loaded up)
110gr Vosler Varmageddon
120gr (I can't remember the name...will have to look it up again. Sig also makes an OK 120gr round)
125gr Fiocchi SST
194gr Lehigh Maximum Expansion (Subsonic)
(There are a few others that are solid, but the Tac-Tx and Vmax are probably the most common)
Interesting side notes:
1. The best .300blk rounds are on the "light for caliber" side, or subsonic. There isn't much of anything above 125gr, and below 175grs.
2. The best supersonic rounds have plastic ballistic tips (Vmax/Varmageddon, Tac-tx, SST), or at least quite pointy tips.
The best rounds seem to be at near peak efficiency already. You try to go heavier, and between the heavier bullet and powder loss, the round falls off of a cliff.