Let'* not tell you what we think of as the best, because the majority would say 97 because we own them.
Let'* tell you what we do know about them. Here'* what I know.
96 First year of the Series 2 motor. Had the 4T60HD transmission. The trans is vacuum controlled and generates a lot of heat. Differentials are known to blow up. Tuning support is through Ryan at GMTuners.com or DHP (no longer produced, but deals can be found). Heads are known to have cracks when magnafluxed. If changing the springs for a cam or rocker install, LS6 springs can not be used due to the spring seat.
97 First year of the Series 2 motor with the fully electronically controlled 4T65E HD transmission. Differential is specific to 97. Has tuning support through Ryan, HP Tuners and DHP. PCM pinouts are identical to 96 year only. Heads are known to have cracks when magnafluxed. If changing the springs for a cam or rocker install, LS6 springs can not be used due to the spring seat.
98 Second year of the S2 with 4T65E HD. Differential was changed and kept through the 4T65E HD production runs. PCM pinouts changed and stayed relatively the same through 2002. New PCM design shows increased buss speeds over 96/97 pcm'*. Different heads that don't appear to have cracking issues. Different head also means the LS6 springs will fit.
99 Much like 98.
2000 Redesigned from the ground up. Many new features, including heated seats that go bad easily. Poor front trans mount that can be replaced/upgraded. No more torque axis mount on the passenger side that makes changing belts tough. Water leak issues in doors from poorly sealed water deflectors. Window regulators break easily. Most items that go bad are easy and cheap to fix. Stabilitrack is a great feature.
2003 GM makes major revisions to the transmission in valve body and supposedly the hardened 4th gear output shaft. Typically the splines on the shaft are soft and over 100K+ can strip out which drops 4th and makes a rebuild necessary. Some early production 03 models had the non-hardened shaft. More updates and better heated seat elements.