Basic rules really only covered a short span, levels 1 to 3, but actually more like levels 1 to 2 if 5E is used as a benchmark as 5E characters have a lot more abilities. The feel of a 3rd level basic PC ismore like the feel of a 2nd level 5E PC. They were very easy to kill with a couple 'good' DM rolls or one big mistake by the PCs. And given the lack of strategydiscussion in that era - PCs made a lot of things we'd call mistakes today.
Expert took spellcasters through 6th level wizard spells, which is more like level 12 in 5E. One could argue the lack of special abilities may equate to a lower level of PC, but wizards, specifically, had a lot more spell slots, longer durations and no concentration mechanic, so they were a lot more powerful. Most players with fond memories of these days are speaking primarily of their experience with expert rules.
Companion (not Champion) Rules took PCs up to the equivalent of 20th level PCs in 5E, but the magic users, specifically, peaked with a lot more power than a 5E wizard due to the amazing number of spell slots and ability to stack spells. My high level PCs from my Companion days used magic to scout before battle, walked in with a dozen spells up on the party, and were my first examples of effective and powerful PCs.
Master and Immortal do not equate to 5E, at all, as they took PCs to power levels that exceed what we see in 5E. 9 spells of each level between level 1 and 9 in Master? However, in practice, there were a lot of TPKs if a DM followed the rules. It was not a well balanced game, and there were too many opportunities for a single bad roll to end a PC. If you had two of those early in a combat, there was no way to recover.