I played Rifts frequently a bit over a dozen years ago, during my munchkin years. My gaming group enjoyed slinging around large weapons, and as a result, every single combat encounter was with the players in full combat gear; there were no cases of "Glitterboy pilot caught outside his armor" and the like. Due to our inexperience, most of our adventuring was about traveling to various places and killing various things; we hardly used the skill system at all. (We usually created characters and played just a few adventures before dropping the campaign, so we rarely got past first level; most of our skills were quite unreliable.)
As a side note, the Rifts combat system is unfortunately boring: the most damaging weapons are all ranged weapons (with a handful of exceptions involving supernatural creatures' punches), so it encourages players to spend their entire time exchanging shots rather than close-combat blows. There are very few bonuses available to hit or to dodge in ranged combat, so every character in the game hits an MDC target on a 5+ on a D20. (Since MDC weapons do 100x damage when converted to SDC, SDC armor is irrelevant.) There are no significant rules for cover, higher ground, sniper perches, or fast-moving targets in Rifts (the best you can do is to inflict a -2 to hit if you run while dodging and weaving), this means that all combat effectively occurs on a flat, featureless plain and there are almost zero important combat decisions to make.
(On a personal note, due to confusion over the rules, my group did allow characters to dodge bullets. This made the Juicer a higher-tier character than my answer suggests: they may attempt to dodge attacks for free, and typically have an excellent dodge bonus.)
My group's munchkinly tendencies, plus the ranged combat system, plus all of us playing at first level most of the time, produced a very simple tier system: How much damage could you do per round, and how much MDC could you absorb without keeling over?
Thus, the Tiers:
Tier 1: Combat characters with 1000 or more MDC. There are a handful of RCCs like this (usually in the later books, where the power creep was just silly), but they're almost invariably banned by the GM. These characters invariably do 3d6x10 MDC or more per attack.
Tier 2: Combat characters with 500-1000 MDC, dealing 2D6x10 damage or more per attack. This includes the Glitterboy pilot and other high-end robot and power armor pilots. These characters are usually not viable in the long term, due to using up their armor, missiles, and other expendables. There are no real rules in Rifts for repairing armor (aside from GM fiat), so these characters have a limited lifespan. (In most games I played in, rewards were small; the campaign usually ended either just before or just after one pilot ran out of armor.)
Tier 3: Combat characters with 200-500 MDC, and either regular damage of 2D6x10 or access to missiles and other explosives. These are usually low-end robot or power armor pilots.
Tier 4: Mages of most stripes. The magic system of Rifts is not kind to first-level characters. Even worse, magic's range is intentionally reduced to ranges that are "more dramatic," while all weapon ranges are more realistic (and usually an order of magnitude longer). They usually use basic MDC body armor and weapons to supplement their magic, which is only better than regular equipment in niche situations.
Tier 5: Combat characters who only have access to basic MDC body armor (around 50-100 MDC) and weapons (around 1d6x10 MDC, or 4d6 MDC if you have no supplemental books; just about every supplemental book had some kind of ranged weapon that did 1d6x10 damage).
Tier 98: Any character with SDC measured in (a handful of dice) x 100 with no access to MDC armor. An MDC weapon will still kill these characters in one or two shots.
Tier 99: Any "normal" SDC character with no access to MDC armor.
For robot or power armor pilots, nudge them up a tier if they have 100 or more missiles; nudge them down a tier if they have less than 10 (and thus their missiles will shortly become irrelevant).
For all characters, if their MDC armor regenerates, nudge them to the top of their tier, or up a full tier if it's going to be a long-term campaign.
As a final note, the number of combat skills available to the character (and whether they were allowed to take Boxing) will nudge the OCC/RCC up or down within a tier. My group lusted after the physical skills, but in retrospect, only Boxing was truly relevant; the others would, in the final analysis, only help with close combat, which we didn't do.