We would recommend Elly’s place. The apartment is big and stylish, with great modern amenities (including a washing machine and clothes rack which is handy when traveling!) It is good at keeping cool in the Seville heat.
In terms of location, it is a bit of a walk from the main tourist attractions (1.5km from the main Seville cathedral and Real Alcázar). However it is still in a cool neighborhood in the old town, and you do get great value for money. There is also a small bus (the C5) which stops outside and can help cut down the walking.