I know I'm probably a little bit biased
but for an "outsider" the Malaga feria is easily the best in Spain.
In April in Sevilla, there is nothing happening in the city centre during the daytime. Everything is centered on the fairground, where, unless you are a local with access to your private caseta, you are definitely an outsider. In Malaga, during the feria del dia, everybody has a ball and the streets are given over almost entirely to the party atmosphere. No comparison.
The one downside is the intense heat, as it is held in the scorching sun of August. Must drink lots of water.... plus a bit of fino and ice cold pale cream sherry too!