Well the Best Mutants in Rifts to use are Dogboys or Psi-Stalkers (both in the mainbook).
Try to stick to SDC based characters at first ... It helps keep the game from going too gonzo right out the gate.
List the OCCs You are most interested in seeing being used by Players ?
I have to say, the Cyber-Knights kickalotofass in a good way. Specially if you use the Master Psychic version listed. Search your Feelings, you know the answer. Beware the Evil-side
The Only characters that might too "powerful" right off will be Full Cyborgs (cause they are mdc), Dragons (again mdc beings), Power Armor/Robot Vehicle Pilots and Glitterboys and SAMAS pilots (the mdc armor and weapons built in deal). The rest are usually great, simple, and fun. Don't get me wrong here, Full Cyborgs, PA/RC, etc are fun, but when you start to need md weapons only the game gets boring fast.
Keep MDC Body Armor and MD weapons to a minimum if you can. If a Wilk's Laser Pistol (1d6 md) is as rare as advertised, when player finally run across someone with one should be scary, not a laugh or walk in the park.
My advice is have the group in a Town, City, area which bans "MD" weapons, Armors, etc. Think about it, Would you want everyone in a town wearing Military Body Armor, and carrying Rocket-launcher scaled damage weapons around ? HeckNo. Nothing you can do about Mages and Psychics who can summon md attacks/defenses though, but you should warn them if they use them, it may be punishable by death by the Law.
Another advice I can give you, is to check out HEROES UNLIMITED 2nd Edition Corebook. It is all SDC setting, has Magics, Mutants, Magic, Supers, Cyborgs, Aliens (space/dimensional), Hardware types, Hunters, Physical Training, Ancient Martial Art Masters, Super-Spies, and more. I USE HU2 Mainly these days with other SDC settings such as Palladium Fantasy, Nightbane, Dead Reign. Mostly because I do not have to deal with the mdc concept of pistols which destroys Tanks in a single blast. The SDC only settings are better "balanced" towards Players and GMs in general. It is less work, and hassle when i do not have to worry about Total party Kills just because i used something that was MD.
I also never liked the idea of requiring the players to use MD gear just hold on to their lives. SDC settings are dangerous like real life enough. Why bring in Ultra-military age items to kill my group, when normal sdc items work just as well, with more drama and suspense.