I will be visiting Seattle for one day, taxi or rental car?

I would not use either taxi or a rental car. The taxi will cost about $50 each way and if you rent a car the parking is expensive and the traffic is very congested.

Instead, I would just take the light rail line from the airport. It is easy, faster, much less expensive ($5 round trip) and less stressful. The light rail line goes from the airport to the end of the line under Westlake Center which is right in the core of the "downtown" shopping area within easy walking distance of the Pike Place Market (about 4 blocks). If you have time, I would also recommend walking down the stairs (or taking the elevator) from the Pike Place Market down to the waterfront.

The Seattle Monorail station is located on the top floor of Westlake Center which runs directly to the Seattle Center where the Space Needle is located. It costs $4 round trip.

If you do go down to the waterfront, you might want to consider walking down to the Washington State Ferry terminal and take a ferry to Bainbridge Island and back. It is a 35 minute crossing each way. There is usually a wait time if you are taking a vehicle, but not if you are a "walk on". You can either stay on the ferry and come back immediately or walk off on Bainbridge and catch another ferry back later.

I've included some links that I hope you find helpful.

Enjoy your visit!

Tori gave good advice. Once you get downtown, you don't need a taxi unless you want to spend all your time looking for a parking spot or sitting in traffic. It's much easier to get around by foot or public transportation.

Light rail to downtown would be the way to go. Once you get to Westlake Center, most everything is easily within walking distance. You can take the Monorail to Seattle Center and the Space Needle. It's only a mile away, so walking is also an option.

If walking is a problem for you, the Seattle Metro Transit bus system has a Ride Free Area, which includes all of downtown. You can hop on any bus and ride for free.

We live in a suburb of Seattle and we never use our car to drive from one place to another when we go downtown. We drive in, park at a parking garage, and leave the car there while we walk. It's much easier!

Neither.

Rail transit from Sea Tac Airport to North Terminus (Westlake Station).

You will be a few blocks from the Pike Place Market.(Go south, away from the Needle a couple blocks to Pike Street. Walk right/west five blocks into the Market).

Ask anyone where the monorail is. It is near Westlake Station and will take you one mile to the Space Needle.

right. taxis are expensive and parking is horrific (yet there are parking garages). Downtown is a lot easier to access if you're just walking around so just take the light rail downtown. But If you're one of those people who likes to drive themselves around, I would definitely recommend a rental car.