Yoshimo seemed to have the worst offense and the worst defense.
Depends on how you use him. Set a ton of traps before battles, and he'll become incredibly useful. I killed Firkraag with just Yoshimo and my monk PC. He's also good at backstabbing due to his high strength. If you don't mind tossing roleplaying out the window, you could also dual-class him to a fighter. With 17 strength and 18 dexterity, he'd be a good flank attacker, but much like with a PC Kensai (unarmored fighter with damage bonuses), you'd have to make sure he didn't get hit too often.
Personally, I love monks, and play them a lot. Watching a character kick someone to death or, better, hit them with Stunning Fist/Quivering Palm, is both disturbing and amusing. I like to use thieves as scouts and backstabbers, but I prefer fighter/thieves or swashbucklers, as they are better in combat--even if the Swash can't backstab. Assassins with their x7 backstab multiplier can be handy too. And it seems like cheating in the extreme, but a dual-classed Kensai/Thief is pretty awesome--Kai+backstab is incredible. If you prefer to hack and blast through battles, a thief may not be the best option; perhaps a Fighter/Thief instead. Dwarven or half-orc fighter/thief with maxed-out strength would be pretty fun

Also, because I was considering taking Edwin as the only other evil recruit for RP, I'm wondering how viable it would be to use a pure Conjurer for melee DPS.
Don't. Seriously. His physical stats are horrible--mage
HP/
THAC0, 10 dexterity, and 10 strength. He will do nothing but get himself killed. He's the best mage in the game (more so if he uses a potion of insight for Wish/Limited Wish), but even dear Eddie can't do everything.
1. Archer (Half-Elf)
2. Wild Mage (Elf)
3. Thief/Monk?
You could download the mod Beyond the Law and take along Kova--half-elf mage/thief--and Kiyone--half-elf archer--instead of having 2 silent NPCs along. Or you could download the
PPG Xan mod and install the optional component to make him a wild mage.
Thief/Cleric is just silly. Until you get Use Any Item, which is frankly ridiculous, cheating, and 4th-wall-breaking, they can't use any weapons to sensibly backstab (because clubs do not *stab*, and no other cleric weapons are "suitable for backstabbing"). Playing one felt like cheating to me.
Ranger/Cleric can be kinda fun. They have access to *all* druidic and cleric spells in the game, and are pretty decent fighters to boot, since they can specialize in weapons and have an automatic 2 points in dualwielding. Crom Faeyr in one hand and the Mace of Disruption in the other is...interesting

I tend to prefer to sneak through battles, backstab and launch a ton of spells, then run away and repeat the process when I feel like it.
There's a lot of fun strategies involving spell spamming. For instance, cast Malison -> Emotion -> Fireball/Chain Lightning/Skull Trap to render most enemies harmless and then nuke them

Dragons can be killed with 3 or 4 castings of Lower Resistance and a ton of magic missiles, or just with a lot of traps, or with lightning spells (all dragons save Abazigal are weak vs lightning). Another popular strategy is to spam a hostile monster with summoned creatures, then send the party offscreen. The boss will then waste their special attacks or spells on the summons, and have nothing but basic attacks left for the party. Great on liches and dragons

It's true that some characters will be able to wipe out enemy after enemy. Try Korgan with his Berserker Rage, or Anomen with a couple priest buff spells (Righteous Magic), or a fighter/mage with Stoneskin and Tenser's Transformation. Little Aerie fired off a spell sequencer in Bodhi's lair in chapter 6, casting False Dawn 3 times and killing every vampire in the vicinity. It just depends on how you use characters. Anomen is best suited to healing and combat buff spells, which can make him very useful in combat. Cernd's high wisdom make him best suited for memorizing a large number of prayers, his Werewolf form makes him a tank very nearly on Korgan's level, and druid spells like Iron Skins make him more useful still.
Sure, 6 illusionists would be funny, but crazy. You'd never make it through most of the tough battles, not without major cheese at least. The same goes for any party of all mages. I had Aerie, Xan, Edwin, PC ranged fighter/mage, Jaheira, and Nalia in the party once. It...wasn't pretty. Jaheira was forced into the front lines (at level 7/7, mind) and poor Edwin was my tank character (due to his 18 constitution). It was incredibly frustrating, because my mages were all getting slaughtered trying to cast spells, and Jaheira couldn't heal them in time.
My ideal party is generally: 1 Tank (Minsc, Anomen, or Korgan), 1 archer (Mazzy or PC archer or fighter/mage), 1 backstabber (Valygar or Yoshimo), 1 healer (Cernd, Jaheira, or Aerie), and 1 fighter multiclass (Jaheira, Anomen, or fighter/mage PC). Since there's some overlap, I normally have: Anomen, PC (elven multi fighter/mage with a crossbow), Yoshimo/Imoen, Jaheira, Aerie, and Valygar. All in all, a very versatile group. Though if you want craziness in the party, take along Jan Jansen and Haer'Dalis