Batman's Alfred Pennyworth vs The Avengers' Edwin Jarvis. Two butlers enter. One butler leaves.
To begin with, I know that I am comparing two pillars of continuity and joys forever. Alfred and Jarvis are beloved characters, and the two should not be in competition. Even if one is a blatant rip-off of another. Just saying.
However, it would be disloyal to the spirit of these two brave and noble men to shy away from conflict, so let's start with:
Both Alfred and Jarvis fall into the typical butler trap of looking middle aged while acting like your grandfather's grandfather on a particularly stuffy day, so it's impossible to compare their ages without going meta. Alfred first appeared in Detective Comics in 1943, beating Jarvis to the page by a cool twenty-one years. That first year, though, was not kind to Alfred. He was a chubby, bumbling idiot who annoyed the dynamic duo by trying to solve crimes and discover their identities.
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Fortunately for Alfred, in late 1943, Batman got his first on-screen role, and actor who played Alfred was skinny and had a mustache. Unfortunately for Alfred, the only way that comics creators could come up with to mimic the show was, literally, sending Alfred to fat camp. He emerged as the thin, mustachioed man we now know him as, but that was an undignified couple of years.
Still, that leaves twenty years of exposure that Jarvis doesn't have.
Jarvis had the initial advantage, what with Marvel copying absolutely everything about Alfred that made him cool, from fighting the Nazis in World War II to the R.A.F to boxing. Marvel also skipped over Alfred's failed stage career when creating Jarvis - a really good move on their part.
However, Alfred wasn't just in the R.A.F. He was a spy. Like James Bond, people, including the hook-ups.
Sure these men starch the shirts of the most famous heroes in the world, but everyone needs a little more to their lives than that. Normally, Alfred would have beaten the pants off of Jarvis in this category, since he hooked up with hot spies in World War II. (James Bond, people. Bond, James Bond, even!) Since that continuity has been lost to a thousand crises, we will have to start fresh.
Alfred might still have won, having been romantically attached to both Leslie Thompkins, a well-respected physician, and to Dana Winters-Drake, a woman half his age. However, Thompkins took a turn for the evil, and during the time of their courtship, Winters-Drake was as nutty as an almond snickers bar, so the field is open for Jarvis.
Jarvis' most recent relationship is with Aunt May. This is a woman who took out at least one supervillain with drugged cookies, and has recently been shown to be worth giving up a marriage to a supermodel. However, since getting involved with her means getting involved with Peter Parker, the world's most self-pitying superhero, I can't see how it's worth it.
Jarvis has never, to my knowledge, been anything less than a friend to the people he serves. On occasion, he has threatened to leave (such as when Tony Stark was re-forming the team and the person who shot him might have been on it), but that was mostly a joke. And while he was skrullified once, that wasn't a failure of loyalty. It could've happened to anyone in the Marvel Universe. In fact, it pretty much did.
Alfred, on the other hand, always seems to have one foot out the door. He has walked out on Batman no less than four times. He has also turned into Batman's nemesis once, after being killed in the 1960's. He came back as the Joker-like Outsider, with a pale face and a nasty sense of humor. Now, one might say that turning evil is the only way to get back from the dead in comicslandia, and therefore not Alfred's fault. But we expect better than that from a butler.
It's a tough one, but I'm afraid that Jarvis just hasn't been around long enough to build up the juice that Alfred has. Alfred is as essential to the Batman legend as Robin. He was even played by Michael Caine in the last couple Batman movies. Jarvis, on the other hand, was just a computerized voice in Iron Man. The winner is clear. Alfred Pennyworth takes the prize.