Best 60-75 gr 243 reloading bullet? Monolithic or not? Best BC?

For what purpose? Hunting and target bullets are built for completely different missions.

As far as accuracy is concerned you are going to have to experiment to find out what your rifle likes. Sierra Match Kings have always been superb performers in every caliber I have tried them in. I would be surprised if .243 was any different.

By the way B.C. is almost irrelevant when you have made a decision to use the lighter varmint class bullets. These bullets are meant for rock chucks, ground hogs, coyotes usually inside of 300 to 400 yards. Sierra's 107 grain Match King will resist drop and wind drift because it's much longer than the weight bullets you have selected.

The 107's also need a 1/8 twist to really shine. I think they have a B.C. similar to a 123 grain 6.5 match bullet. .530 +/-.

This is for target, also i need to stay under 75 grains as my barrel is a 1:12. The v max look good, anything else? Also this is for extended ranges, does bc still not matter? I would imagine it would

Berger's 65 grain BT is a match 6mm bullet with .281 BC.  It stabilizes in barrels as slow as 1:14.

Berger also has an 80 grain 6mm varmint bullet with a .300 BC specifically for 1:12 barrels.  This is the highest  Berger 6mm BC for 1:12 twist. Its a flat base design, so it has a slightly lower BC than the boat tail V-Max, but Berger FB bullets are incredibly precise.

Berger has five or six different 6mm bullets that will stabilize in 1:12 barrels.  All are laser beam accurate.  Berger does not make bullets that are not championship accurate.

Monolithic bullets like Barnes all copper, may not be a good choice for your slow twist.  Being long for weight, they usually require tighter twist to stabilize given the same weight as lead core bullets.  I like the 80 grain TTSX for hunting with my 6mm Remington, but I have a 1:9 twist barrel.