O'brien is my favourite character, so I may be a bit biased, but he did manage to keep Deep Space Nine (a cardassian station) up and running without being fully equipped to deal with it. I've not seen enough Enterprise to make an informed desicion about Trip, and B'Lanna was interesting, but as an engeneer I don't think she's up to O'brien standards.
EDIT: Sorry about all the spelling mistakes there, but I just got up
the thing about o'brien, imo, is that because he wasnt an actual officer, they were never going to make him chief engineer of a federation starship... but ds9 is different, and when they got the defiant, it just happened to fall under his jurisdiction...
Gender: Male Location: +40° 36' 5.70", -73° 57' 49.46
B'elanna Torres
Voyager had a harder time, no starfleet no other ships...but i love o'brien...i just i remember him most as the transporter operator on enterprise.....his work on Ds9 was great though