I have Exage 300 lx crank 48,38,28 Crankset. It is also Biopace. I would like to replace it since it is worn out. What current model from Shimano is a suitable replacement?
You don't need to replace the entire crankset; just the rings. But since the time and effort you put in to seeking and buying the rings of the right bolt circle diameter (BCD; the size of the circle of the bolts that connect the chainrings to the 'spider'), which for your cranks is 110 for the middle and largest ring, and 74 for the inner ring. You might find it simpler to source an entire replacement crankset.
The number of teeth is up to you, but don't go way outside what you originally had otherwise you run into problems like chain length. However, when one replaces chainrings, it is often time to replace the chain, and cassette too. You should check the wear of the chain by either using a chain testing tool (you can get them on ebay for about 6 bucks) or by using a ruler. Two links should be exactly 1 inch, so over 24 links you might see some stretch if it goes beyond 12 inches. Just what is acceptable stretch you will have to google, but certainly there's the risk of your chain not fitting on the new chainring if the chain and the old chainrings wore each other out.
As a rule of thumb, a chain is replaced first, and then every perhaps 4 or 5 chains, if you replace it on time, you will need to replace your cassette. Then, after maybe 4 cassettes, your chainrings will also need to go.
Have you replaced the chain and cassette recently?
Also, the new rings need not be biopace, but if you do get biopace, follow the instructions closely on installation, because they go in a certain orientation with respect to the spider.