That question is always loaded, because the answer can vary in so many ways depending on who is asking.
Instead, here are some things that are on the forefront of my mind as "good gear that presents a unique value for some reason". I don't own most of it, and I don't intend on buying it, but for the reader it might be of use.
Suhr SL68: if someone asks about what Marshall to get, this is actually the first thing I think of. It's built right, the right components, the right component values.
... but then, I also think about the new 20 watt Marshall Origin. It's less than $700, sounds like a plexi, has a couple of bonus features that are well thought out. And it's less than $700. Enough power for any situation short of an arena. Less than $700.
Egnater 15 watt Tweaker. Still the most useful all around tube amp, plexi/Fender/AC30 tone stack/path, less than $500. For most probably the best choice if one isn't sure what one likes or wants.
Boss 50 watt Katana: yes, a gazillion people have already written about this. I had not heard one until last week, and I was impressed. These will eventually be what I use at my office for lessons. They've got it dialed in perfectly. While the tone control response is non-traditional behaving, and somewhat compressed, in reality that's more useful in most situations than what the "real" version does. All the effects you'd ever need. $210, 50 watts, plenty loud for, again, any situation. And it doesn't weight anything, I think it's actually levitating, so for a "toss in the car" kind of gig it should rule. I hate, hate liking something that has received such overwhelming mainstream acclaim, but it's deserved.
Yamaha THR-10: the Almost Katana. I have one of these because it's more portable than the Katana and because it's stereo, more conducive to being used as a portable writing/recording situation. I believe Yamaha's circuit emulation is pretty spot on, the downside of the amp being the tone controls are very exaggerated sounding and can get you into trouble too easily. The bizarre thing is the lack of a speaker output jack; if I could easily plug it into a 1x12 cab these would be what I'd use for guitar lessons. The little speakers means it sounds "thin"; but the recorded-via-USB sound is very good. But the important thing (that Boss gets right with the Katana as well) is that the touch-sensitivity/feel is "realistic cranked amp". It can be tonally manipulated by touch, something most modelling amps can't do. It's accomplished here because they're "modelling" the circuit path instead of just doing a "snapshot" impulse response. I believe that's also why the Boss works as well, because it's Roland's COSM analog-circuit emulation.
Used Blackstar HT-1R: this is what I've been using for guitar lessons, does Fender/Marshall with the turn of a knob. A positive thing about it is that they do the Apple product thing in that they don't give you much choice to allow you to get into trouble: just a tone control. It has a 1 watt tube push-pull output that through a 4x12 sounds very, very authentically plexi. The reverb sounds very good in it as well. I paid $89 for mine on a Guitar Center "Daily Special" deal; if these sold for $100 new I would recommend these to any new student automatically. It can do any style of music from country to the metuhls.
Fender Deville: a cheap way to get Tweed Fender sounds. Not quite perfect, printed circuit board, weighs a lot. But a lot cheaper than a reissue. But not as authentic (but I prefer them; I wish I hadn't gotten rid of my 4x10 because it was killing my back getting it in the car as my clean amp).
Fender Deluxe R.I.: most common all around amp. The Hot Rod versions are good. For some this would be the last amp they'd have to buy. Another amp I wish I still had one of laying around if I were rich.
Where are the boutique amps? "Boutique" amps are effectively just better built versions of various Marshall/Fender circuits. A person taking advice on amps shouldn't be spending money in this category! As far as I'm concerned, the Suhr Sl68 IS a boutique amp, but it's not *that* much more than a reissue, and I can definitely say - having owned maybe a dozen vintage Marshalls - that it's the thing to get. I could maybe say Matchless would be the thing to get for an AC30 enthusiast, or the U-2 Edge/church gig guitarist, but in reality it's actually *better* than a real AC30 and in turn sounds clearer/brighter. Fenders - there are so many variants that it is it's own minefield of decisions.
Get a Boss Katana if you don't know. Otherwise - dive in! It's an interesting field, learning what your favorite guitar player actually uses, on what recordings, and figuring out if that's what YOU want. It's a learning process of discovery about YOU, your preferences and what YOU want to hear. Be warned it's a very deep thing people spend their lives studying, researching, and exploring. It can quickly become a blackhole of info, that in reality the Average Listener can't consciously discern a difference between. BUT, if YOU can hear it you'll play better and have more fun. That's what it's about, right?