For me (and we've had this discussion in one form or another before on the list), the single most important measurement is apparent top tube length. For sloping top tubes, I measure this along the horizontal, from center of head tube to wherever the center of seat tube/seat post would fall. If it isn't close to 57 cm ctc, the bike won't fit for long ride comfort.
If that's in the ballpark, and I could ask for one other measure, it probably would be standover height. Either it is stradle-able or not.
Seat tube length? To me, it's just a way of estimating whether the bike is worth going to look at. My "fleet" runs from about 56 - 62 cm seat tube ctc, but I think all are 56 - 58 top tube ctc. Even the 73 (?) Cinelli SC, at 62 cm+ seat tube, is only 57 cm top tube. Great ride for an old guy: bars are a bit higher than usual.
couple of other measures: If I'm going to commute or tour with it, I want chain stays at least 17" or ~44 cm. Pannier clearance. At my age, I also like toe clip clearance relative to the front wheel. Otherwise, it's all about who designed and built it (angles, etc).
harvey sachs mcLean va.
Bob Hanson observed:
The more I considered this issue the more it seems that there no longer is no good way to "equalize" frame measurements between bikes with single statistics.
Cannondale had been (and maybe still are) measuring their frames to the top of the seat tube... but this top point even on road bikes had extended considerably above the top of the top tube - and that was with level top tubes on their bikes. -- Measuring either c-c or c-t does not take into consideration the drop of the bottom bracket. -- Stand-over height also depends on the size of the tire in order to compare frames, so perhaps measuring with bare rims would be better... but how would we account for differences between time-trial bikes with 19 mm tubular tires and city/hybrid bikes with 38 mm clincher tires -- "stand-over height at center of top tube" would help compare compact frames with lowered rear triangles... but it does not account for the length of top tubes, so I would not know if I'm comparing a virtual 60 cm with a virtual 52 cm frame. And, even comparing "virtual" level top tubes as a basis could result in my comparing my 62 cm 1950s randonneur frameset with your current 52 cm Trek racing bike.
Anyone have an idea for a NEW perfect system?