Hi Glen,
Interesting stuff -- I missed that one on the website and also missed the papers. Thanks for that.
Yep, they are certainly right up there among the candidates for the most massive/most luminous stars of the Milky Way. NGC 3603 is a very, very massive and bright cluster -- one of the brightest and most massive open clusters known in the Milky Way. Seemingly inconspicuous Westerlund 1 (on the border between Ara & Scorpius) is also of a similar nature and maybe an even more massive OC. There is a paper on Westerlund 1 here:
http://arxiv.org/PS_cache/astro-ph/p.../0504342v2.pdf
Several other contenders for Milky Way's brightest star include:
Eta Carinae (of course)
HD 93129AB which is very close to Eta, at the heart of Trumpler 14 within the Eta Carinae complex. This is a binary where the combined mass is estimated at 200 solar-masses and the larger component is likely >120 solar-masses.
The Pistol Star
LBV 1806-20
Cygnus OB 2-12
At the moment, the last mentioned is apparently ahead of the others by a whisker, though it is a really difficult thing to pin-down the exact brightness, magnitudes and masses of these "megastars" given the uncertainties in distances and extinction due to dust and other "variables" in the modelling of "top-shelf" really massive stars and their evolution.
All these stars including the ones you mentioned are in the same cricket-ground weighing in with somewhere between about 100 and 140 solar-masses and bolometric luminosities (ie across all wavelengths) approaching 6,000,000 solar. One study suggests LBV 1806-20 may be as bright as 40,000,000 solar, though that is in at least some doubt -- other papers suggest it is binary or multiple and not a single source.
Interestingly, HD 93129A which is currently classified as O3 Iab has also in some literature been classified as O2 Ia star -- the only one known (if that is a correct classification -- it is disputed). It is one of my favourite stars in the sky. Secretly, I hope it wins the contest! It has the extraordinary surface temperature of 51,000 deg K!
I hope Cygnus OB 2-12 doesn't win -- it's a (the only) northern hemisphere (Booo ... ) contender.
There is some further info on all these exotic beasties (and a couple of others) here:
http://www.tim-thompson.com/bright-stars.html
Best,
Les D