As others have said, depends on what you want to see.
I-84 - Nice views of the gorge, but longer and kind of out of the way.
Hwy 26 - Great view of Mt. Hood and then once you get on Hwy 97, great views of the Sisters Range.
Hwy 20 - Nice view of Mt. Jefferson and a few high mountain lakes. You also get to go through Sisters, which is a nice little town and some good views coming into Bend.
Hwy 58 - Not a bad road, but not a whole lot to see either. The tunnel is closed about 50 miles from I5 between 8pm and 6am Mon-Thu. This might be the quickest route to get to the north entrance of Crater Lake though due to the lack of towns and a reasonable drive on I-5, probably the least mountainous.
Hwy 138 - Long drive down I5, however nice views along the umpqua river with lots of waterfalls to stop and hike into. Could also swing by Diamond Lake.