There's little debate: the Coast Starlight is the best route that Amtrak offers. Spanning more than 1100 miles from Los Angeles to Seattle, the Coast Starlight offers its passengers a delightful range of scenery, from
crashing waves and rolling hills to snowy mountains and thick forests. Along the way, the train makes major station stops in Sacramento, Oakland, and Portland as well as dozens of other cities and towns that span the West Coast.
The Coast Starlight is a long-haul Amtrak line and offers a full range of standard amenities, including a Sightseer Lounge with Cafe, full Dining Car, and baggage service. Superliner Coach seats are quite comfortable--comparable to what one would find in First Class on most international flights before the advent of the lie-flat seat--and the large windows in the Coaches as well as in the Lounge give you ample opportunity to enjoy the landscape as it passes by. You'll also find a mini-arcade on the lower level of one of the Coach cars; this is the only Amtrak route to offer one.
With so much to be said for Coach, you might be surprised to learn that the Coast Starlight is also the route on which you probably benefit the most from choosing to upgrade to First Class, or what Amtrak calls "sleeper accommodations." There are a variety of sleeper options from which to choose, but the most affordable is the Superliner Roomette; on the Coast Starlight, these cost around $100 (per room, not per person) above Coach fare. (True Bedrooms are also available but cost much more, often around $400.)
Roomettes tend to be a good value on any Amtrak long-haul train; in addition to showers, fresh linens, and the comfort of a true lay-flat bed, your meals as a First Class passenger are included at no cost, which saves you $10-15 per meal per person. The Coast Starlight is a particularly great route on which to upgrade, though, because you also get something extra-special: the Pacific Parlour Car.
The Parlour Car is a unique treat. On one level, it's not so different from the Sightseer Lounge: there are places to sit, tables, windows, and a nearby cafe attendant. But the chairs are plush and more comfy, and the cafe offers complimentary coffee as well as reasonably priced cocktails and specialty coffee drinks. In the afternoons, there are wine-and-cheese tastings for $5--the wines are forgettable, but the cheeses are often impressive--and the lower level is a movie theater.
You'll also find tables in the Parlour Car, and you can choose to eat any or all of your meals there as an alternative to the main Dining Car. The menu is limited but specialized--nothing on the Parlour Car menu is available in the Dining Car, or vice versa--and the service is less rushed and better suited to couples.
Oh, and did I mention? The Pacific Parlour Car is exclusively for passengers traveling First Class.
All tolled, a trip from Los Angeles to Seattle (or the other way around) takes two days, leaving on the morning of the first day and arriving at around 9:00 p.m. on the second. You'll spend one night and enjoy five meals aboard this Amtrak flagship route, and tickets can be as little as $100 per person for one-way Coach. Add the Roomette and you're looking at less than $300 for two people--a small price to pay when you're also enjoying scenery and service like this.