I always wanted to "3.5" Rifts for better play but never did.
Juicers I remember were powerful. Beyond that... I played only once but bought about a dozen books to use then the group decided they didn't want to try something other than DnD. Aw well.... it's just money (I can say that then since I had unlimited OT and nearly no bills)
M&M is the best way to do the 3e version of this. Alternatively you can go wild with d20 modern or True20, but you'd have to not care about balance if you're oging all the way by including dragons and psionics, etc.
Most powerful Combo: I remember the space book having a race with "gods" as the function of it, like if you wanted to play Thor or something. Spellcasters would know and cast every spell of a particular level: first would get all 1st level spells, second level you'd get all the 2nd level spells, etc. I don't even think it was if they took a spellcasting class, it was like a feat or trait they could choose on top of anything else.
By craziest combo, tho, are you talking about PC combos? In that case, it depends on what resources you're allowing. If I could be, like, a splugorth or deity, sure I'd be that. But if we're talking core book... that uber psionicist would be a good start.
Juicers are cool, and the whole "you die in 5 years" assumes the campaign will last five years (ugh; that's like assuming Elven lifespans will affect anything other than roleplaying). But it depends on the gear and situation and suchnot, and what the other players have.
for example, if the Juicer is gear with weapons and such but is on the ground, and another player is a dragon or deity and can shoot them from space with MDC damage... or just teleport them into the Sun... well, there you go. I'm sure some grognard or other has a character designed to max out teleporting a character into the sun.
juicer-wraths are NPCs, tho, so I don't think they count. Same thing for monsters like splugorth; or most of the options mentioned (octo-guys, etc.).
And Vampires were pretty powerful, I recall, unless you had wood, silver, or a squirt gun (which was silly, but that's Rifts for you).