By a long shot, Sensaphonics 2MAX. Runner up, Etymotic ER4P.
I give a lot of in-ear recommendations. Right now here’s what I’m telling people: If you want a super cheap pair around $100, grab a set of Shure SE215s. Don’t buy anything for any more than that until you hit the $300 price point - then grab a set of Etymotic ER4Ps. Don’t buy anything for any more than that until you hit around $1000 - then drive up to Chicago and grab a pair of Sensaphonics 2MAXs.
There’s my 2 cents. For more like 75 cents, keep reading…
For about six years I’ve been trying different in-ears. I’ve demo’d or bought ears from Westone, JH Audio, Livewires, Ultimate Ears, Futuresonics, Shure, and Etymotic. I was looking for something that would be flat enough to give a realistic idea of what was coming out of FOH while performing and to do some mixing if I wanted to, but also exciting enough to keep me engaged while performing. Here’s what I found:

ER4P - $290
In the $300 range, the best ears I’ve found are a universal ear called the Etymotic ER4P. (The Edge from U2 uses them if that lends some more credibility to them.) They’re flat and clear. To me they feel a little much in the highs and a little lacking in the lows, but as a vocalist that’s typically how I want it anyway. They feel accurate, clear. Using these after my previous ears (a set of Westone UM2 universals) was like taking a blanket off of my head. The clarity is amazing. It’s also nice to not have to mess with molds. I highly recommend these. My only complaint is that when performing live, sometimes they tend to underwhelm me. I hear everything (which is more than I can say for most ears), but I don’t always feel it the way I’d like to. Still, I’d rather hear everything than listen to a set of ears with no clarity and overhyped bass (most of the others).

Sensaphonics 2MAX - $850 + ear impressions = $950
After demoing 6 different JH models (even buying a set of them), the Futuresonics offering, and purchasing a set of Livewires, I can say very confidently that Sensaphonics is where it’s at for 3 big reasons:
Sound - They’re flat, but still huge and exciting. Overhyped bass is fun for a few gigs. Then it just gets tiring - and it screws with a vocalist’s ability to find a pitch reference. I want good solid bass, but I want it to be flat - like the mix or monitor engineer intended. These sound beautiful. Huge. Wide. Full. Clear. They’re the closest thing I’ve heard to the Etymotics, except with better bass and a slightly more balanced sound. They also feel “deeper” than the Etys, if that makes sense. Not just lower. Like there’s more space to look around at what you’re hearing.
Fit - This is a huge one. Bigger than I expected when I started this quest. I had over 6 re-fits of my Livewires purchase - with about 2-3 weeks in between each new re-fit. I can’t tell you how annoying that was. I had about 7 or 8 refits of my JH Audio ears. They were much faster and way better to communicate with but I still never got a good seal, even after 4 different audiologists and impressions. Yuk. With Sensaphonics, I took one trip to Chicago, sat with Dr Santucci to talk about hearing, had a hearing test done, had the impressions done, and in 2 weeks I had PERFECT fitting in-ears. Obviously having Sensaphonics do it in-house was a big part of getting a good fit, but they’re also the only manufacturer I’m aware of who does soft silicone molds. Every other custom ear I’m aware of uses a hard silicone. I boggles my mind as to why this is. When I sing, my ear shape changes. Soft silicone moves with my ear canal and keeps the seal tight. Not only is this good for audio, but it’s extremely comfortable.
Customer Service - When you buy a set of universal ears, customer service isn’t that important. Unless they break. But when you buy a set of custom ears, many times tweaks are necessary. Working with a company who answers the phone is very important. Between Thom and Dr Santucci, I couldn’t have asked for a better experience.
I know it can feel like a fortune when you start getting into the $1k range for “headphones”, but if you’re a musician who makes a living making music $1k is probably pretty average AFA key gear purchases go. Your guitar probably cost that much. Your keyboard probably cost 2-3 times that much. While instruments and mics and such are important, in-ears are what you perceive your reality through. To me, ears are one of the most crucial pieces of gear you’ll ever buy. Here’s my advice: Buy the best ears you can. Do it once. Then move on and make great music.