of course some were closer than others, but the way you want to qualify 'the best terran..' in this discussion is highlighted by the fact that they couldn't win an msl/osl.
instead i'll start by laying out which players impressed me the most and left an impression even though they ended up losing in those matches which would have crowned them or gotten them closer to their first title.
midas
solid player. had some great chances towards the end of 2005-2006 when he was easily a top8 contender.
left an era of fans and hopes alive by staying active towards the end of 2009-2010.
upmagic
had a rock solid zvt and was above average in the other matchups. iirc, he had a more difficult time in one of the two remaining matchups which would cause him to drop out of tournaments pre-ro8 while he was playing at his strongest.
was very imaginative and had cryptic play for a short amount of time, which left me to see him as the terran nal_ra.
iris
who could forget this man's 2-0 over ggplay before losing in 3 straight sets to immaculate (and hard to top, to this day) defense plays. iris was a very strong overall player leading up to this point in his career. his style was very apparent and difficult to play against leading him to change his ID to what fans had given him for a nickname.
out of anyone, i believe iris is the terran who literally came the closest to a major (without having won one beforehand), and fell just short.
sea
had the ability and star sense to, but could not break the ro16 to ro8 threshold to claim a title.
imo, lacked just a bit of polish to be able to come close to a starleague championship during the middle portion of his career.
hwasin
was extremely strong in individual leagues for a short stint in his career.
lost 2-3 to savior in a semifinals where his next opponent would have been bisu, had he won instead.
light
though not as strong during the majority of his career as would be indicated by his generic looks and nickname (invisible terran) i actually had fond memories of him coming oh-so-close to beating jaedong in a semifinals.
believe it or not, i heavily favored light going into that one. his TvZ record was off the charts at the time.
goodfriend
as much as you guys give this guy flak, seriously, he was the most consistent ro8er. top 4 finisher in multiple majors.
he lost to july in a finals 2-3 in and around 2005 where july was at his peak in terms of domination.
out of everyone out there, i still have to give it to goodfriend. he may not be memorable to you guys, but i saw this guy leading up the finals more times than you can count.
and as the amateur scene was bustling and new faces came out to reap the benefits of the matured practice regime and methods which took roots in the olden days (hello flash, jaedong, stork), competition was stiffer, and you needed to be extremely strong with no bias in your style to be able to compete with every single big boy that was seeded or was qualified to play against you through a stiff wildcard/dual-tournament. that is what it meant to be the best for any lengthened timeframe. each terran i listed above made a big impression on me, but ultimately could not finish the deed.
i can never say they deserved the starleague, because preparation or conditions said otherwise.
some players who were in the same boat matured and eventually claimed their title, if only their one and only.