Just Starting

[Sentinel]

I am currently running a PFRPG campaign. Once this wraps up, I plan on starting HU (first time).

So far, I have picked up the core book and the Mutant Underground. I was just wondering what other books are useful as well as where I can find character sheets for copying.

I recommend the HU GMG, Powers Unlimited One, and Powers Unlimited Two, depending on what your budget can support.
The GMG is a very useful tool for Quick Roll NPCs, Q&A and Errata on powers and power combinations among other things, Super-Brawling rules, additional weapons and equipment, and a couple of good introductory scenarios.

Powers Unlimited One is very nice for the new powers.
Powers Unlimited Two is good for additional Power Catagories (character archtypes).

I use a modified Wormwood character sheet, which I did some cutting and pasting to.
I believe PUII has a generic HU sheet, and there are sheets at this site as well.

[RockJock]

Sentinel has the right idea. PUI and II are needed to finish fleshing out powers and classes and the GMG helps with villians and combat. Century Station and Gramacy Island are good books if you need help with a setting. If you are creative, or have read lots of comics you don't really need them.

[Sentinel]

Plunder all other Palladium books as you see fit.

Take Apoks and Holy Terrors from Wormwood.
Use Rurel Eel People and Tritonia from Rifts: Underseas.
Adopt the Kydians from Rifts: Atlantis.
Make extensive use of The Megaverse Builder (Dimension Book Seven).

Got plans for a Space Based campaign? Use Phase World, Mutants In Orbit, Three Galaxies, and Macross II.

Want to spice up HtH combat beyond the old "strike/parry/counter-strike/parry"? Ninjas and Superspies, and Mystic China.

I can't tell you how many villains and villainous henchmen came from Rifts: Mercenaries.

Rifts: Psyscape to amp up all your psionic needs.

The Palladium Megaverse is your playground.
Just don't pee in the sandbox.

[The Artist Formerly]

In order of importantance.

1) HU GM's guide. A ton of useful stuff for gaming in there. Guns, magic, some adventures, NPCs, guidelines for balancing super heroing and secert identities, ideas for handling the wide array of things that come up in game.

2) Powers Unlimited One. This expands the number of super powers available to your game and is a really useful resource.

Those two give you a good bump to add to your existing resources. Everything else is just 'extra'.

Villians Unlimited is a book that provides a few GMing resources such as government sponsored super human control teams (SCRET), and a whole lot of villians for your heroes to smash the crap out of. It's handy to have, but not having it isn't any big deal either.

PU two was kind of disappointing. All it adds are more OCCs, most of which already existed in the HU book, just with a slightly differant bent to them or minorly differant background. You could have a player find a class s/he just falls in love with, but not having it, won't impact your game by any means.

Century Station and Gramercy Island are linked as setting books, both describing the same general areas. If you need setting help, Century Station is friggen awesome. I love the two books and use them a great deal, however, once again, not having them, won't impact your game.

Aliens Unlimited and Aliens Unlimited:Galaxy Guide are only really needed if you intend to run a game with deep space as major setting backdrop. If you want them, great, if not, eh.

[Sentinel]

Aliens Unlimited and Aliens Unlimited Galaxy Guide can also provide useful hardware if you find you need more potent weapons.
There are genetic enhancements, guns, robots, armour features, vehicles, and other things not found in the HU main book or GMG.
Also, additional powers not found in Powers Unlimited. Some of these are only really good for a space setting, but some of them can be useful simply for more powerful characters as well.

[Puroresu]

Here are some recommendations I have for your Heroes Unlimited Campaign:

Nightbane is good if you want it to be dark, Gothic, horror-style a la DC Comics' Vertigo format.

Villains Unlimited is really good for some quick villains. Gramercy Island is great and so is Century Station.

For dimensional travel, definitely purchase The Megaverse Builder!

Aliens Unlimited, Galaxy Guide, Phase World, Mutants in Orbit, etc. are all great for a space opera campaign.

Pantheons of the Megaverse is great if you want to use mythology. In other words, gods like Thor, Herakles (Hercules), would work out very well in an HU campaign.

You can't go wrong with Mutant Underground, especially with the latest X-Men movie coming out to give you even more inspiration.

Powers Unlimited One and Two are good. The HU Gamemaster's Guide is also good for reference material and GM ideas.

Rifts: Skraypers is great for playing super heroes in outer space.

[khyron1144]

PU two was kind of disappointing. All it adds are more OCCs, most of which already existed in the HU book, just with a slightly differant bent to them or minorly differant background. You could have a player find a class s/he just falls in love with, but not having it, won't impact your game by any means.

I have PU1 and PU2. I like them both a lot, but if I had to choose just one I would actually go with PU2.

The OCCs seemed to be the thinnest area of the HU2 main book.

PU2 supplies classes that allow you to build a character a lot like Swamp Thing (plant Gestalt), The Swarm (animal Gestalt), Firestorm ( psychic Gestalt), Hourman (Imbued), Popeye (Imbued), Green Arrow (Ancient Wepans Master), etc.

[RockJock]

The books you pick is really determined by what kind of game you are going to run. I'll freely admit the first few HU games I ran were total Marvel conversions, but it gave everyone the hang of running and playing HU.

[Sentinel]

I converted a lot of my original characters from Marvel into Heroes Unlimited, when it was possible to do so.
In some cases, I was forced to re-invent characters as I had to find Palladium archtypes and abilities to support the characters. More often than not, I liked the way they came out.
In time, new psionic powers, super powers, invocations and magic orientations all came about to increase my range of character creation, and made me a happy camper.

[RockJock]

Ziggy is totally correct. The main HU book is more then enough to play a full campaign. All the suggestions are good books, and would be more or less usefull depending on how you play your game. The setting and feel you are going for is a big factor in what books you will need.

[Sentinel]

For myself, I know that when I started with HU, I wished there had been more powers available.
The Power Catagories were fine, but I really wanted more varieties of powers.