I have to say Negima because that is the first shonen i have read/watched. Thus making me like the rest too. Negima is so touching too! *sniff sniff* (not saying that the others weren't)
Dbz was the first shonen that I read. But there is a large number of shonen I like. Claymore, love hina, elfen lied, full metal alchemist, etc. Sums up my childhood in a nutshell.