What are the highest level quest mods for BG2?

lollers

No matter if the quests have strings attached, or are part of an NPC package or a new land, I will not discriminate ona any basis other than experience level. I like to play with EET, so that gives me a larger world map to play with than just the few same places every time in ToB.

gorgonzola

I am not sure that I have correctly understood your question, as I am not a native English speaker, but IMO the mod that gives you the most difficult side quests, and change some vanilla quests in something a lot harder to beat, is the Tactics mod.
Some of the quests are more single encounters, like the lich in the docks, the acid kensai and the red badge, and the only real quest, that you can accept or decide to avoid, is the tactics ritual, but the battles are really hard.

depending on the level you do them some other mods also offer quests that can be pretty difficult, it comes to my mind the Tower of Deception mod that is tough if done at low levels and the Tashia (mod npc) personal quest that you have to do later, but also has a very good battle.

lollers

You sort of read it correctly, and I'm sorry for not speaking clearer in the first place. I find that ToB has a dire lack of content. The mods that work for SoA are fun, but SoA isn't like ToB, it is enjoyable without them. Only mods I've heard that do anything about it are reunion, and a couple of other mods that are harder to use safely such as the longer road and wheels of prophecy because they are just a few mods out of many that can modify the final few hours of ToB. I did enjoy going back to have another go at the SCS improved drow army of Ust Natha after my little confrontation with Irenicus in hell because EET will let me do that. So I'm not going to exclude SoA mods on that basis as long as they're suitable for the top level characters.

gorgonzola

The Red Badge and the Tactics ritual are done in the WK area, so can be done at ToB level.
The first triggers when you are in the WK external area and after a certain xp treshold, that now I don't remember, and is basically a single tough and interesting battle in a very small custom area, you are tele ported there and... you have to survive.
The second one is given by a mysterious person named Pontifex the first time you reach the WK external area, you can accept it or refuse it, but afaik not delay it. If you accept you are moved in a large wilderness area where you have to stop a ritual, and you have limited time to do it, so basically you can not rest. There you have to defeat several groups of enemies, that have a pretty good AI script, so even if no one of them is super strong you need to kill fast and to be enough tough to survive their attacks. As you play SCS it is probably easier then intended as the insect spells are one of the tactics used by those groups, but SCS make easy to counter it, still should be a good challenge.
If you choose, or if you fail the mission, then you can fight against Pontifex himself that reveals to be a powerful creature. Other way you get his reward.

I don't want to spoil you any more then this, but you can download the tactics mod and read the documentation. Feel free to add or avoid the SoA components, the final SoA battle should be compatible with ScS, as @JuliusBorisov play it with that setting.

About tactics and EE the mod is now without maintenance and the conversion is in a beta phase, but should be playable (I play it with my original game, so don't have personally tested it with the EE verson. More information about here:
https://forums.beamdog.com/discussion/41242/tactics-mod-bg2ee-compatibility-conversion-and-beta-test

I hope it helps.