I've never actually played an evil party in either BG1 or BG2. I rolled a sorcerer last night, but I'm not exactly sure what NPC's to pick up. I'll take neutral or evil characters, and I want characters that either transfer to BG2 or have some type of plausible reason why they wouldn't appear.

The only two NPC's I know to pick up for sure are Edwin & Viconia. I typically play the old Paladin role, so I'm looking for something different.

You've already mentioned the two npc's I think of as must-haves.

You'll need a thief of course. The possibilities are as follows:

Montaron
A decent halfling fighter thief. He comes with Xzar who you could either use in place of Edwin or push off a cliff. Both Montaron and Xzar are referenced in BGII.

Shar-Teel
Shar-Teel can dual class to a thief and makes an awesome backstabber. Her drawbacks are she's so good at backstabbing, you'll be tempted to put most her points in stealth and neglect spot traps, and then you'll still need another thief, plus you have to weight if you want to level her as a fighter first. While she doesn't appear in BG2, there is a minor plot twist involving her in BG1.

Tiax
He comes late in the game, and is another mage-thief where you already have two spell casters. Still, he does make an appearance in BGII.

Safana (neutral)
Pretty much an Imoen replacement. You could use her until Shar-Teel duals.

Skie (neutral)
Available only late in the game.



You'll also want some muscle (in addition to Shar-Teel if you use her) and Kagain is great for that. He doesn't appear in BGII, but I can't think of anyone else so would still recommend him.

So perhaps the PC, Edwin, Viconia, Shar-Teel, Kagain, and one other thief type?

For an evil party I strongly recommend using the BG1 NPC Project mod, and have Montaron and Xzar in your party.
Xzar is not the best choice from a powergaming POV, but he has some unique and fun quests added.
Montaron is a Fighter/Thief, so in addition to the fun stuff the mod adds to him, he's also an excellent character for your thiefly needs as well as for backstabbing.

Shar-Teel and Kagain are excellent evil aligned fighters.

So I'd choose my 5 companions from these six:

Monty
Xzar
Edwin
Kagain
Shar-Teel
Viconia

And also temporarily add Tiax just for the lulz, and then drop him. With the NPC Project mod he has an interesting quest in the city of Baldur's Gate.

Just to clarify:
The only *playable* evil/neutral characters that transfer to BG2 are Viconia and Edwin (and using BGT their stats and memorized spells transfer too).

I agree with RickTaylor's assessment.

The first evil playthrough I did on BGT I took Viconia,Edwin,Shar-Teel,Montaron&Xzar (I had a thief (assassin) PC though so Monty's thief skills were backup.

2nd time I dropped Monty&Xzar and took along Kagain and Imoen (I know Imoen's good but it amused me that she might be the one person my evil character might give a stuff about (this time anyway, last time she was dropped asap) - and maybe he's drawn to keep her close as she's got that connection - currently on the BG2 part of that playthrough and planning to rescue Imoen later )

With a sorceror PC you could bring along another mage but I wouldn't bother with 2 more so I'd go with Edwin and the party RickTaylor suggested adding in Safana as she's a pure thief.
(NB: Skie needs you to have Edorem (sp?) in your party and to bring him from cloakwood to BG city - bit of a pain for a late-game thief whose development you can't control so much)

If you've got the BG1-NPC project installed then Monty+Xzar become more recommended as their banters are amusing, Tiax is similarly amusing but I didn't find him all that useful (played with him in a neutral playthrough with Quayle in the same party - they hate each other).

edit: I'm a bit slow typing - PetrusOctavianus made the point about the NPC project too

I'd have to reinstall everything to add this NPC Project, wouldn't I? Not that re-doing the BGT install is hard, it's just kind of a pain.

I'm not sure from memory where in the install process it comes but at the least you'd need to uninstall the tweak-pack as that needs to come after. Doing a full reinstall might be safer...

NPC project is good - adds the banters, some little quests and romances for Shar-Teel, Branwen and Dynaheir and Ajantis, Coran and Xan for the girls. It also adds a few little extras - (all parts are optional installs). It's not essential by any means but I'd recommend giving it a try - read up on it here and see what you think:
http://www.gibberlings3.net/bg1npc/index.php
(links on the right for the readme and the forum)

I found the NPC project to be excruciatingly annoying, with the interjections all over the place, virtually in the midst of battle, and NPCs saying random crap I don't care about, but I know I'm in the minority there.

I didn't reinstall, just added the NPC project to my existing. Seems to have worked well.

Getting Shar-teel sucked. I've never recruited her before, and had no idea where to look. I ended up having to reload atleast a dozen times. Though I did use Corax the Ghoul to kill about four Lesser Basilisks to level up, so it wasn't all bad.

My party right now is my PC, Imoen, Motaron, Xzar, Shar-teel & Kagain. I want Viconia for sure, and probably Edwin, so at some point I assume I'll dump Motaron & Xzar, even though Xzar's come in pretty handy so far.

I'd keep Monty and drop Imoen.
Monty is more suitable for an evil party and much more fun than Imoen, and with two spell casters (protagonist and Edwin) a Fighter/Thief is more useful than a Thief/Mage (which is the optimun career path for Imoen) IMO.

One thing you should remember if you're bent on playing evil is not to let your reputation drop too low.
In BG 2 it only makes items more expensive, but in BG 1 having a very low rep will basically make your game unplayable by spawning endless amounts of Firefist mercenaries to attack you.

For evil parties, as others said, Edwin and Viconia are the only 2 that transfer, although their BG 2 versions have better stats, so you may not want to transfer them anyway.
Personally I like Xzar and Montaron so I'd recommend them, and you can keep the last slot open for doing the NPC quests in the NPC project, or pick either Shar-Teel or Kagain for more front line power. Note that Kagain is a better tank, especially if you give him the constitution book for 20 con, but Shar-Teel has a greater connection to the story.

In BG2 you can get guardsmen, Cowled Wizards, and Radiant Heart knights attacking you at (very) low rep in Athkatla, but they won't spawn infinitely the way they do in BG1. So it's something to consider there too. Really though the whole reputation thing is kinda silly. Why do evil party members WANT to be hated? Do they have some sort of complex about being popular? You should always have the highest possible rep that won't cause party members to quit, as rep gives a bunch of random dialog benefits as well, the only tradeoff is that you have to listen to your party bitching (for whatever reason). Neutral party members bitching at high rep makes even LESS sense, but at least they won't quit.

I'm monitoring my reputation pretty closely. I'm trying to roleplay a evil sorcerer who was always outcasted in Candlekeep. Both due to his natural magical abilities (Sorcerer), his hidden background (Bhaalspawn), and the mystery around his adoption. They'd treat him nicely, but he could always feel the fear and suspicion from them. With the idea being that now he's free to explore and exploit his natural powers. I'm going with the definition of lawful evil being exploitation of power to one's own ends and goals, not "murder every civilian you see".

He only cared about Gorion & Imoen (taken from TrollumThinks above), so he wants to avenge Gorion, and treats Imoen well if gruffly, but his secondary goal is power and wealth. If by solving the crisis, he can get closer to the killer and gain power, he'll play "hero". I'm down to 7 reputation after the NPC project kicked in and Motaron killed the distressed father in Bereghost, and I sided with Silke. I then did the returning of Jolia's ring, and a donation to try and raise my reputation.