I'd say it's the one the thumb is on. However, I've heard 98-00's don't have a bleeder bolt.
To properly get bleed/burp the air out of the coolant system, follow these instructions:
1) Remove the tank cap
2) Set heater temp on high (85)
3) Let engine warm up to normal operating temp (to open the thermostat)
4) Rev engine between 2000 and 2500 RPM for 5 minutes (at some point you may see some fluid "blurp" (technical term) out as the air escapes)
5) Turn off engine and let engine cool
6) Add fluid to the tank as coolant is drawn back into radiator (don't let tank get empty)
7) When engine is totally cool, add fluid to full level
8) Replace cap
Oh, and make sure your thermostat has the bleeder/jiggle valve at the 12 o'clock position.
95 LS400 Pearl White
90 LS400 Pearl White R.I.P. 11/2008