boulder, co to portland, or end of may

I'm riding from nj to portland and have my route planned out with a few rest/family and friend days in the denver area. I know there are several passes through the rockies, and I was looking for some advice and/or recommendations for the best routes. I'm riding alone at this point, anyone in the area who wants to and can at the time, please feel free to join. Thanks in advance for any info.
Ryan