The best or very good?
The Canon 50mm f2.5 is very nice for getting close-ish. It's sharp at every aperture and doubles as a decent portrait lens. But when you need to get super close, getting light on the subject becomes an issue.
The various Tamron 90mm f2.8s are very nice for the price, esp if you buy used and again, makes a nice tighter portrait lens.
Either of these can be bought used in great condition from a good source for $200-ish or for closer to $350-$400 new.
But if you want the best, the Canon 180mm f3.5L is what you want. The 100mm is more versatile and adds IS, but the 180 is where it's at.
To do serious macro work, you will need a good tripod and a decent lighting setup.