Green Alder is the best in my opinion.

Some might cringe at this, but what I do is run my chainsaw until it is out of oil. Make a few cuts off the log end. Then take a round of alder and rip it (with the grain) the long shavings that come off a nice new blade make some real good smokin.

When you have enough just put the bar oil back in your saw a run it again.

Wet Luhr and Jensen chips are a good substitute.