A pal just gifted me the legendary edition of Skyrim. What sort of mods would be considered "must" to make this game playable these days? As far as I know Requiem/SkyUI/TrueCompass/BetterQuestText gets you already far but is there more essentials?
Picking mods is a huge academic topic that it's impossible to teach or learn in a single forums reply or even one whole forums site. But here I can give you some general advice that I've learned from a whole month of intense modding training:
Stability Is Everything
You must have some basic knowledge about the game before doing any modding or you'll have lots of trouble facing frequent CTDs/Infinity Loading Screens/freezing and not knowing what causes what:
- Know that the game WILL crash when its memory usage reaches 3.1 Gb. You MUST install ENBooster to fix that.
- Know that the game WILL ILS/freeze when it loads lots of textures and stuff. You MUST install Safety Load to fix that.
- Know that the game's OFFICIAL esm files contain so called "dirty edits" which may ruin your modded game in the long run. You SHALL use TES5Edit to "clean" any dirty mods.
- Know that the game WILL crash if you don't have a proper load order of mods. You MUST use BOSS to manage a proper loading order (strictly speaking it's not necessary as your experience increases, but it'll still save a lot of work).
Scripted Mods Are ALWAYS Used With Caution
Know that unlike Morrowind/Oblivion, in Skyrim there is NO official "clean save". For a lot of mods which use scripts, if you uninstall the mod, then any game saves that use them will have references or orphaned scripted still embedded in them. Even if the mod author states that uninstalling (in a proper way) is safe, some references will still be there. If the situation is bad, in the long run it will cause save game bloats (game save size reaching hundreds of Mbs) and/or constant CTD or glitches (e.g. werewolf form taking minutes to set off)
Know that in the case above, but in a worse sense, a LOT of mods can NEVER be getting rid off, especially for mods that the author stopped support for a long time.
Know that a lot of random CTDs and glitches are caused by using a lot of scripted mods. Because of the nature of scripts, they require constant updating, and because of the sucky papyrus engine (the script engine of Skyrim), running a lot (6 or more) of heavily scripted mods will cause resource misallocating or conflicts or shit that will simply glitch out your game. Examples:
- Wet & Cold's script will do a check whenever you switch zones. I CTD'd a lot when I use it during zone transitions. I also got one vanilla game event failed to trigger while using Wet & Cold because the event occurs exactly as I change zone. Uninstalling W&C fixed that.
- Enhanced Blood Textures have scripts that cause NPC to make blood pools. The script will make a dead NPC gush out blood every few seconds and apply multiple stacking blood texture on the ground he's laying on, simulating a blood pool. You can imagine how stressed the papyrus engine will be if you killed a lot of NPCs and therefore a lot of blood scripts are running simultaneously.
To avoid script related stability problems:
- Always read the mod description (how heavy scripted it is, how is its compatibility, etc.) ESPECIALLY if it has any instruction on how to UNINSTALL it.
- Always check the latest updated file's date. You don't want to install something that's last updated in early 2012 or something unless it's just textures replacement.
- Always check the latest forums posts of a specific mod. Sometimes the author have important messages stickied on top. Sometimes you'll see other posters complain about possible conflicts or CTDs, and hopefully provide guidelines or fixes. Sometimes you'll read from posters that the mod is abandoned by the author and such mods shall NOT be installed unless you know what you're doing (once again, exceptions are simple meshes/textures replacement mods or mods that other posters deem ok to use).
- Always include scripted mods one by one (or three by three), do not throw in a dozen scripted mods all together and declare the game "STABLE!" just because you don't crash when you reach Riverwood. There stands a good chance that you WILL crash when you go to Whiterun, and by then it'll be too late (or at least very tedious) to fishing for culprit mods.
PS: with a lot of mods with scripts you can disable specific functions to relieve the papyrus engine's stress. For example you can disable blood pools of EBT, the "water dripping" shader of Frostfall/Wet & Cold, and so on. Unfortunately a lot of mods or their functions require periodically checks (a few seconds or so) and you can't disable them; using a lot of these mods will cause trouble. How much is too much you may ask? Well, I can't give a clear number because apparently there's a kind of "threshold" of game stability. Try to install 5 scripted mods, play for a few hours, then add another one tomorrow, if it's stable add another the next day, and so on.
Overhauls Aren't Necessary The Best
Despite what others tell you, SkyRe or Requiem are not strictly must-haves for your game. The main problem with these mods is because they cover so many things (from perks to game mechanism to damage and so on), it will make your game far less moddable, so if you're not happy with one aspect of the mod and want to use another mod to replace a function, your option will be either very limited or even impossible. For example, want to try Apocalypse Spell Package, or Deadly Combat/Deadly Dragons? But will they balance well with Requiem? Thankfully both SkyRe and Requiem are both self-moddable to a certain extend. However...
Which brings the greatest problem: such mods are not supposed to be casually installed or uninstalled midgame. You're supposed to start a new character with them and stick right through it. If you find the mod not to your liking after 30 hours (perhaps you hated some changes, perhaps you wanted some other mods but they're not compatible, and so on), then you must uninstall the thing and restart. It's not that restarting is bad for this game anyway, but it can be frustrating for you if you're in a mod trying mood.
Personally I prefer to use a combinations of mod choices because I know what needs to be modded and to what degree, while SkyRe and Requiem can bring a very unique, possibly "a different Skyrim" experience because they really "overhauled" everything. I didn't choose the latter also because I'm still fine-tinkering with game stability issues and gameplay balance understanding, so using self-choices gives more room to experiment.
... it's useful for me to test all Features
pseudo medieval monk from oblibion instead of guy clothed in ancestor monks; install ''Timing is everything'', set the timing of vampire attacks on 100th level and enjoy the Requiem which adds all useful assets like crossbows to the gameworld. 
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