Choosing the right thermal paste

I'm building a PC and just was wondering if I should get  artic silver, ic diamond, or mx4.

IC Diamond > MX-4 > Silver

I've been using Arctic Silver 5 since I started building really. Have used it on quite a few builds as well as on a GPU or two and my Xbox 360. Works very well. Isn't too stringy and isn't ridiculously expensive. 

I do believe AS5 is slightly better than MX-4, but MX-4 has the huge advantage of being non-conductive.

All are good thermal pastes, though.

+1 for non-conductive.

Gelid extreme is the best, I use IC diamond myself but I heard it causes damage on cpus and baseplates so watch out if you go IC diamond!