The TransAm will give you the best cross-section view of America, with a little bit of everything, exploring the diversity. The TransAm has a lot of cycling infrastructure in place, which makes it very good for first-timers. There are a lot of free camping sites along the TransAm, something you may not find as plentifully on other routes. I did the TransAm on $16 a day. Most of my other routes have been at least twice that. I always recommend the TransAm for a first-time tour. With your comments on boring, however, I'm not entirely sure that it's right for you. Personally, I found Kansas quite interesting, but not everybody does.
For scenery, your second route cannot be beat. So if scenery is your top priority, the Pacific Coast and the southwest US are good choices. The Pacific Coast has plentiful and cheap hiker/biker campsites. All of the national parks you mention also have camping, although not usually as inexpensive.
People are a very important part of bicycle touring enjoyment for me. In my opinion, the middle of the US is best for this. Small-town America has a charm that's hard to beat, and the people are amazing friendly and generous. I wouldn't want to miss this. Also, the TransAm is a very popular bicycle route, so you'll meet many other touring cyclists on that route, which is fun for me.
Every tour has its own charm, and it's a very personal decision.