Well, Berkeley and others say 1L performance gets most of the weight. Stanford says you must have been a "competitive" applicant from undergrad/masters program in order to receive serious consideration. But, "competitive applicant" can mean many things.
Plus, most of Stanford's applicants are people the adcom would at least consider admitting out of undergrad/masters. They just don't get many ex-cons with 2.5/145, no softs and so-so LOR's. lol. The quality of the school you are transfering from, however, can also take on serious weight, depending on the target school. Thus, with a few notable exceptions, it's almost always better to transfer from a so-called "top-50" school.
One positive with Stanford is, they do not care what your reason is for transfering; they assume you are looking for an "upgrade" and they are fine with that.