Depends on Domestic/International AND level of elite status.

DOMESTIC

All airlines only upgrade elites if there is space still available they don't think they can sell, often close to the flight time.

For 25K/year elite flyers:
US and NW, because this level is too low to get a high percentage of free upgrades on any other airline. US sells few F seats and NW tends to have a larger ratio of F making upgrades more frequent to 25K/year flyers on these airlines than others. The US and NW F product also tends to be a little lower quality than the competition. (DL, CO, and AS are the other airlines that offer unlimited free upgrades to 25K/year flyers. AA and UA also offer a limited number of space available upgrades to this level of flyer.)

For 50K/year elite flyers:
US, NW, DL, CO, and AS all offer an unlimited number of space available upgrades at this level. Of these, CO is the least likely to have unsold seats left to upgrade you but also tends to have the best domestic F product. UA and AA also offer a limited number of free space available upgrades as this level and you can buy more. On premium trans-con routes, AA and UA have by far superior premium cabin seats. On other routes, with the exception of those domestic routes operated by international aircraft, AA and UA offer a similar F product to everyone but maybe US, which is generally inferior in quality. AS would be a logical choice as this level if your needs match their route structure as you would now have top tier AS status at 50K/year of flying.

For 75K/year elite flyers:
NW, CO, and DL are the winners as you will have top tier status on these airlines but not on AA, UA, and US.

For 100K/year elite flyers:
AA is the clear winner as this level. Unlimited domestic space available upgrades with a clearance rate of 75%-100% depending on your route and days of the week. This is a higher clearance rate than on UA, which also has their top tier at this level but does not offer unlimmitted domestic upgrades. 1Ks get 500s and CR1s wich combined will cover you when flying domestic if you do a mix of international and domestic but you will run out if flying mostly domestically and have to buy more 500 milers to still upgrade. US, the third airline with a 100K top tier, offers an excellent clearance rate but the product suffers, making them a less than optimum choice at this level of flying.

INTERNATIONAL

For 25K, 50K, and 75K a year flyers:
- AA and UA are tend to have better clearance rates using miles to upgrade than DL, CO, NW, and US. Upgrades can be cleared right up to the last minute on UA and AA if there are seats available; not so on many others.
- AA advantage: lower fare categories qualify for the upgrade than on UA, but AA charges a $ copay of around $300 in addition to the miles.
- UA advantage: miles only needed, no copay, but one needs to buy up to mid-range fares to be upgrade eligible; low fare classes won't work.
- NW Miles only required to upgrade, but only highest fares qualify. B and Y fares, often several times as much as cheapest available.
- CO Lower fares than NW qualify, but they charge very high co-pays (ie $450 each way).

For the 100K level flyer:

- Only two logical choices:

AA - you get 8 certificates/year than can be used to upgrade most fares including cheap ones. Of course, it is space available only, for any AA trip up to three segments anywhere AA flies, including long haul international ie Brazil, India, Asia, Europe. Unlike free domestic upgrades, these system-wides can be confirmed at time of booking if there is upgrade inventory available.

UA - you get 6 certs/year than can be used to upgrade most fares but not the very cheapest ones. Of course, it is space available only, for any UA trip anywhere UA flies, including long haul international ie Asia, Europe, South America. Unlike free domestic upgrades, these system-wides can be confirmed at time of booking if there is upgrade inventory available.

DL give a number of international upgrade certs good only on very high fares.
US give a couple of fare resricted certs.