Bogota is one of the best destinations in the world to see street art. There is even a free street art walking tour in the city which attracts dozens of people every single day. The street art is colorful, enormous and super symbolic and meaningful. The free walking tour provides the much needed explanations to decipher the various works.
Valparaiso, which used to be an important port city before the opening of the Panana canal has a very alternative feel, and therefore attracts numerous street artists. You will find works of street art all over the city, from building walls to painted right into numerous flights of steps. As this is a super hilly city the street art on the steps is particularly popular and really pops as you walk up to it.
The best place in Santiago for street art is the Bellavista Barrio. One of the reasons there is so much street art in this area is that the shop owners themselves will hire muralists to paint the buildings so that they are more attractive to those who walk by! The artists use anything they can get their hands on; markers for sketching first, and then spray paint and regular paint to complete the artwork.
Another alternative city, Berlin is riddled with street art, especially the political kind. If you're short on time a visit to the East Side Gallery is an absolute must.
In Bristol you can see street art by the world famous artist Banksy. As this is Banksy's birthplace you will see many of his murals all over the city - one of the most famous being the naked man hanging from the window, hiding from his mistress's husband.
London is an enormous city so naturally there is a lot of street art to be found. There are numerous tours taking you around to the various neighborhoods where you can see the best works of art, though you can also find these on your own with a bit of research. While street art used to be part of the underground scene, nowadays it is more a part of the mainstream.
Lisbon is where you will find the Underdogs project, best defined as an international platform aimed at creating space within the contemporary art scene for artists connected with the new languages of urban visual culture. Although you can also find street art in Lagos and Porto, this is nowhere near the scale you can find in Lisbon.