Nidalee's winrate actually tanked. Even though i do feel like she was buffed, her playstyle did change a lot + she's a bit harder. Once people start playing her top as more of a bruiser (she's an awesome anti-melee like Rumble and she has powerful gank potential like Rengar since her passive has 5000 range) her win rate will start going back up. She's going to be a great AP toplane splitpusher, she's just not quite as good as a "spam nine million spears" poke champ.
Remember: her spear damage got nerfed by ~10% and they were made somewhat harder to hit (they are about 2/3rds the thickness of a Mundo Cleaver). But the rest of her damage was increased by ~35%.
She's going to be weaker if you play "human-only spam Q" Nidalee of old. i do think her assassin matchups in midlane actually got improved since Pounce is a powerful tool and she starts with cougar level 1 which prevents the level 2 all-in from Fizz (her biggest assassin counter at high Elo).
Skarner is completely absurd atm. His winrate shot up 4+% and is still climbing.
i need to go spam him in ranked.
His only real weakness is he is kinda shitty very first clear unless you get a good leash. But you just have to use special runes/masteries to make it work. And his build paths are not what you'd expect. He does clear with decent health, but he's not a power farmer and will run low on health if all you do is farm, so you should focus a bit on ganking.
His new passive helps him donate blues, and it prevents a tiny bit of damage early game. It's not particularly powerful early ganks, but Skarner's new E helped his pre-6 a lot. And you never build damage Skarner wtf. He does so much damage with just a Sheen and Elder Lizard since he has the second highest base AD in the game and spams Sheen so easy, and his Q is an AoE physical damage machine. Plus, his ratios are like the worst in the game now.
0.4 AD with an additional 0.2 AP for the second+ Qs
0.6 defensive AP shield
Luckily his Q is on a short cooldown so the ratio isn't quite as bad as it looks since you'll cast it twice as often as most other champs' Qs.
The nerf to his CDR passive did hurt his ult/W spam but they did drop the base cooldowns a little. So he won't have such a long cooldown if his team is losing really badly so he can't use his passive.
BotRK is still going to be a weaker laning item. It was buffed against tanks but nerfed against squishies. The only redeeming aspect it has is that it's a bit cheaper so you might finish the entire item against a BFS + Vamp and have the advantage there. But Cutlass still loses against BFS and Cutlass + double daggers is still probably weaker than BFS + Vamp.
BotRK still scales really well though.
Right now you've got a few options... For early-game items you can either:
1- BFS > BT/IE is still best if you can afford the BFS. Vamp > BT is an awful build path. IE is so expensive + shitty build path so it's really hard to build that in a reasonable amount of time. i think you have to give up any 15 minute dragons because the enemy ADC will have a completed item and you won't.
2- Cutlass into BotRK. This is really good if their team is super tanky or you see their top getting fed.
3- Double Dorans into Trinity
4- Triple Dorans into IE (really only viable on Cait, imo)
5- Brutalizer into IE/Trinity (with a sustain support and/or lifesteal masteries)
6- Essence Reaver if you fall way behind in lane.
Vamp as a first-base is really really bad now. For 800 gold you get 10 AD and 8% lifesteal. 2 Dorans for 880 gold gives you 14 AD, 6% lifesteal and 140 HP. The Dorans might not build into anything but Triple Dorans against a Vamp means Vamp loses lane and gets completely snowballed on. It's super-critical that you win lane and take tower + dragon if you go triple dorans. Going triple dorans and losing lane probably means you won't ever catchup.
If you don't want to go triple dorans and are going to go Trinity Force/BT, you can go buy a second dorans + longsword + 2 pots for second shop. That is 870 gold for 15 AD, 3% lifesteal, 70 hp and 300 health in pots. And 360 gold towards your final build.
Ghostblade is really good now as a 3rd item if you're fed, scale hard off of AS, and the enemy team isn't building armor.
Dorans into Trinity makes you a midgame monster. And it helps the historical problem of Trinity Force where if you are sitting on only two components of Trinity, you're really weak statwise and you have no lifesteal. Plus, Zeal got made cheaper. i know that Corki is picking up a lot of steam for this reason.
BotRK will be good on those end-game AS-ADCs who don't like Last Whisper that much anyway and might go Ghostblade (Twitch, Kog, Vayne). It helps them shred tanks even more and gives them that peel. BotRK + Stattik + Ghostblade is the Twitch dream for massacring tanks.
i think you're better-off generally avoiding Essence Reaver on ADCs. It's a decent item if you fall way behind (it's 800 gold cheaper than BT/IE/BotRK, gives lifesteal, and a decent amount of AD. Mana-on-hit + CDR really helps you be able to continue farming as Ez/Cait/Lucian/Kog/Varus, etc).
It's going to be really good on mana-gated AD champs such as Jayce or jungle Khazix. i can see it as a cheap replacement for Hydra for those kind of champs.