For me it is a tie between BG1 and PT.

Initially I had very mixed feelings about PT, found the setting a bit weird and confusing and did not like how few items there were (particularly weapons). But then I was probably expecting BG1 in a different setting. And once I adjusted my expectations, I fell in love with PT. The writing, quests, the story, the setting, attention to detail, are all incredible. I feel like a lot of games these days try to emulate that (with lengthy dialogues and item/scene descriptions) but none come even close.

BG1 on the other hand is a bit of a different animal. But I loved the freedom of exploration it offered. And there were so many characters with which to build your party. Finally, Baldur Gate city itself was amazingly rich and continuous so I really felt like I was exploring it (in contrast to Athkatla which is just a few disconnected areas). And I also really enjoyed the plot, which not only unravels slowly but is also, in my opinion, really cool (Sarevok trying to engineer the war between Amn and Baldur's Gate to become a god). Finally, I liked the artistic choices made.

By far the weakest of the IE games was IWD2. Felt like it was excuse to slashing hordes of enemies and I found the plot very weak. And leveling was terrible as you were double-penalized when you leveled since you were getting less experience from killing enemies. That makes many fights really a chore since there was nothing to gain from them.

Regarding IWD2, there is a way to counter-act the slowing down of leveling.

The XP you gain is based off the *average* party level. So, if you keep one character at level 1, or add a new level 1 character to the party, your entire party will gain more XP. Be aware that this *can* break the game, and is in fact what I consider a severe flaw in the game.

In IWD1 classic, you can add a new level 1 character to the party without cheating and not break game balance (the character may take a bit to reach a decent level, but if you do this at single digit levels, the character will end up less than a level behind), but if you try that in IWD2, the rest of your party will level up too fast, and the game could easily become too easy. If you want to add the new character without ruining game balance, you *have* to cheat the character's XP up to the level of your old characters, and then level up the character.

What they *should* have done is this:
* XP gains should be based off each individual character's level, not the entire party's level, so that adding a new character doesn't break game balance.
* Dungeons and Dragons has traditionally had a rule where you can only gain 1 level at a time, and would lose any XP that would put you past a second level up. That rule should have been implemented here. (On the other hand, the Gold Box games implemented this rule, and they really should not have; XP doesn't depend on level, and there are times where you might not get to a place to level up for a while, forcing the player to waste XP, which is quite frustrating. The start of Curse of the Azure Bonds is a particularly awful example here; the player can dual class for the first time in the series, but you are right away thrown into a dungeon you can't leave, preventing you from leveling up, which in turn means that most of the XP your newly dual-classed character gets down there ends up wasted.)

Yeah, I know this work-around. But it feels superficial and, as you wrote, it can easily break the balance of the game. I found that a really awful game design...

As I said, it's really a combination of two factors:
1. The 3e D&D XP distribution rules are flawed. They didn't break the table top game because usually the PC's are about the same level, but they do break IWD2.
2. D&D has the rule that you can't gain 2 levels at once, which would have prevented this from being game-breaking. IWD2 does not implement this particular rule. (The other IE games don't implement the rule there, but it's not a problem there because level doesn't affect XP gained there.)

Temple of Elemental Evil avoids this issue by using separate XP calculations per character (as the 3.5e rules specify), and I believe Knights of the Chalice does as well. Keeping a caster one level behind to earn extra XP (for crafting items) is a viable strategy here, but won't completely werck game balance. (Although, in KotC, you can craft items at parts of the game where there's no campfile, so you can't rest; this means that, if you're out of healing spells, you can just craft some scrolls out of nothing (or wands) and just use those; similarly for low level offensive spells.)