Just wondering which airlines programs offer complimentary upgrade easily?

Complimentary, to those with no status, I'd say it rarely happens.

I mean those with status.

This question is confusing. All the legacies except American offer upgrades to all elites on a space available basis. How frequently you get it will depend on your status and the routes you fly/time and day of week

This is for USA bsed airlines. Foreign carriers as a very general rule don't give free upgrades.

Yes, but I think the question is asking which airline gives them most easily, all other things being equal.

For example, if you have the 50,000 mile status on UA, DL and AA, and you fly LAX-JFK on each at the same day and time, which airline is most likely to upgrade you?

Yeah, that is the question I am asking.

Anyone mind to share your insight?

Share your flying patterns, destinations, types of tickets bought, and people can tell you more. As it is, your question is like "which airline is best?"

best program

DL, since AA and UA don't offer complimentary upgrades on JFK-LAX (UA doesn't offer them at all, AA doesn't offer them to anyone below EXP, and 50,000 mile status is not EXP). All things are NOT equal in this particular case, on this particular route.

That being said, the question of "Just wondering which airlines programs offer complimentary upgrade easily?" is so vague and unclear we might as well be discussing how many angels can dance on the head of a pin. It's not answerable without knowing things like what status level, what route, what time, what airline; all of these things affect chances of upgrade.

Exactly. Or "what is the shape of water?"

OP, what precisely are you after? Because saying "Airline X offers the best chance for upgrades for 100,000 Kryptonium �berelites on the LIT-ELP route" is pretty useless trivia if you're not going to fly to ELP or LIT. If you're not actually asking about a particular situation, but have pure intellectual curiosity about some vague "what's the easiest airline to upgrade on" concept... it's really an impossible question.

Since no one seems willing to provide any information, I will start.

I have had colleagues who were low-level elite on US Airways and got upgraded far more frequently on cross-country routes than those with the same status on larger legacies. Perhaps the small size of the airline makes it less attractive for corporate contracts that often fill premium cabins.

It does happen on flights, including international flights, in the case of coach oversells as well. I have been upgraded multiple times on BA long hauls from premium economy to business as a result of being and EXP. I have also been double upgraded from coach to business on an instrument and then from business to first due to oversells in business on UA and I think on AA once as well. All were related to being top tier on the carrier or their partner.

Delta ... but their miles will be worth much less ... so in a way its not *really* complimentary since they make up for more upgrades by giving you less.

AA might be also a good choice since you can redeem 500-mile upgrades with much more certainty.

I only have experience with Delta and USAir. I tried CO just as they were merging, so I don't think that is an accurate evaluation.

US tends to be better for most flights, as they have fewer elites to compete against. That being said, their planes have fewer F class seats, so popular routes (i.e. transcons) are a little bit harder. AA has very few F class seats on their planes, but they use a coupon system. You pretty much know when you are going to get an upgrade, but it will cost you! DL tends to fly larger planes with more F class seats. This works to your advantage when you are flying unpopular routes, but because they have so many elite and are such a big player, and hub to popular destination (such as SFO or LAX or JFK) you will be pretty low on the list.

So yes, it does matter a lot on where you are flying. DL in particular is also susceptible to time - lots of contractors fly Sundays and Fridays, so those days give terrible chances at upgrades on DL. I think US tends to get fewer of those, so I notice day does not make as much a difference.