The red one. Seriously. Apart from the tons of crap I got following the links to the others, I have a 2nd gen 6i6 and it’s well-made and sounds as good as you’ll want. Better, if you’re as old as I am. 
 

TBH, the sound’s good enough on all of them, but the present generation of Scarlett gear is great value and quality. And red. 
 

OTOH, my name’s Poppy. Could be unconscious bias going on here … 

Thanks for the feedback. That's what I thought, but I wasn't sure since I've never done this digitizing before.

CherylJosie - saying "cost easy on the budget" doesn't make it simple for us to suggest other interfaces you might look at. What is your budget limit?

Of the three you mention, I would probably put them in the order of Focusrite, Presonus, Zoom for sound quality in typical usage. However, if you had, for example, a low-output microphone such as a Shure SM7, you should certainly consider an interface that had more gain than any of those three.

On the other hand, you also say you have "several small mixers". Are you planning to plug your microphone(s) into a mixer and then feed the output of the mixer into the line inputs of the interface?

BTW, don't decry the 192KHz specifications of a mixer. Putting enough care into an interface design to get it to give low noise and low jitter at 192KHz can result in improved performance at lower rates. This applies as much to parameters such as interference generation and susceptibility as it does to the quality of the audio signal. I design these things, and find myself spending as much if not more time on the unseen aspects of the specification as on the audio path and the computer interfacing. Operation up to 192KHz is often a natural consequence of a good design, whether or not the Marketing Department demanded it.

Part of the problem here is my relative inexperience despite my advancing age. Although I'm also a design engineer like you, I'm not a recording engineer or a DSP/conversion person. I understand the technical issues from a hand-waving perspective only, especially now that I'm off work with disability for over a decade. The calculus is long gone.

I'm on a fixed disability income and tightly budget constrained. Nothing I do could be remotely considered mission-critical or well funded either. My optimal budget is $0 but I'll stretch it as much as required to get something that won't break after one year and won't frustrate me with bugs, quirks, or serious design shortfalls.

I'm hoping to keep it under $200 for the audio interface, but even better if it's under $50. Anything that I can find readily on Craigslist, Ebay, Reverb etc. at a substantial discount with an excellent chance that it still works properly is a plus.

I have several low budget and frankly archaeological microphones such as the obscure Astatic Blue Line 'Pro Series' cardioid dynamic microphones marketed to kiddies in the strip mall music stores of the late great 1980s:
https://reverb.com/…

BL920(?) 'vocal microphone':
https://reverb.com/…

BL94(?) 'instrument microphone' got two of these:
https://www.musicin…

These microphones have a bright sound even with full proximity effect. I'm expecting their resonance isn't properly tamed with competent frequency-dependent damping. They also don't match each other well in timbre so the 'pro series' moniker is an exaggeration given that one of the two 'instrument' microphones has noticeably pronounced treble/sibilance. I bought them new for about $50 each IIR in ~1984 and they are practically like new because I never used them.

There's a third microphone model that may have been quality at one point but it's on life support now. I inherited it from a friend who gave it to me in the same 1980s time frame and it was already old. Aside from the low frequency rolloff it sounds okay but I haven't tried using it again since the pop filter turned to dust when I opened the case to inspect it and see why the switch was pushed in (any idea of what I can replace that pop filter with?):

Electrovoice 671 with a crumbled internal pop filter and pushed-in switch to avoid accidental off mode
https://i.pinimg.co…

The dynamic microphones are primarily for ensemble band practice and will usually plug directly into a mixer, unless something in my thinking changes after I try the next (preferred) approach.

I also have 6 Behringer B2 Pro large diaphragm multi-polar condensers that I'll be using to record acoustic guitar, small percussion, vocals, talking head diatribe interviews etc. in my bedroom. These are my best studio microphones and obviously with a stereo interface I can only use two at a time unless I use a mixer but probably won't ever do it. They have internal low cut that I've never used even when recording a live loud bar band from right in front of the stage but I did use the -10dB pad to ensure I didn't clip them. I previously borrowed a Zoom R24 for those live recordings and aside from the hassle of running cables everywhere it worked great.

Apparently the B2 Pro patent and manufacturing facility was bought from a pro microphone company, but the electronics may be re-engineered for cost reduction and possibly leave something to be desired. The only shortcoming I've noticed is that the tone gets much thinner in bipolar mode, but cardioid and omni sound okay, so I'm guessing that if they did change out the electronics, the thing they jettisoned was the EQ and maybe some dynamic range. I've found that if I mirror the published frequency response with an inverted line filter function in Audacity, it seems to fix the thinnish sound well:

https://www.behring…

These four microphone types are likely all that I will be using in the immediate future since I already have them and I'm not in a position to invest in a lot of gear.

Then there's a host of battery-operated portable condensers/lavs and various passive dynamic microphones including a boundary mike that I inherited from someone who decommissioned a small commercial recording studio. They seem equally old and definitely not close to pro quality but I may investigate them out of curiosity eventually.

Everything is up for grabs because there's no substantial investment yet in anything except the guitars that are likewise budget:

Marcus Miller 5 string active/passive bass 18V
Yamaha 4 string bass
Ibanez 4 string fretless active bass 9V
Martin Sigma 6 string acoustic guitar with bridge pickup and 9V active EQ
Takamine acoustic guitar
Fender Player strat, recent model, classic strat bell-like tone (real strat pickups, alnico?)

Add in a budget keyboard and we've got nearly everything I would ever plug into this interface in the foreseeable future.

When I said minimal cost I meant it. I'll only be paying extra if durability, usability, or functionality seriously merit an upgrade.

I like that the Focusrite is in a metal box and has a good reputation. I'd prefer MIDI but it's not essential since I don't have anything else that is MIDI right now. What the Focusrite lacks IMO is built-in recording. That's something that I'd like to have eventually, preferably multitrack like the R24...someday.

If I do use a mixer it would only be for live performances (practices in the garage really) or as a work around to latency issues. I prefer to keep the signal path as clean as possible. For mixers there's an ancient and somewhat noisy Tascam 8/12 channel without phantom power in the garage, and a failing Soundcraft '7 channel'/12 in the bedroom/studio.

Also there's a Mackie 4/8 with simplistic effects, a Behringer 4/8, and an Ammoon 12/14 with failed effects available cheap/free depending on the mood of the guy who is unloading them. So far I haven't felt moved to buy and there's another party in line with right of first refusal.

There's an Alesis rack/pedal effects module that I might use too available from the same guy who is unloading the mixers. In that case I'll probably have to use a mixer or expand to multichannel that I probably can't afford, unless I use the effect channel in an amp, and then I'll probably be whacking the S/N considerably while maybe sacrificing stereo. Dunno, this is all new to me and likely doesn't matter at this time.

I hope that helped. Thanks for your input.

The B2 is built into the original casing for the Rode NT2 mike but the guts are all different. Rode’s later NT2a has a different casing AND innards.

Have you sorted an interface yet? I’m probably going to move some of my stuff on, including a Scarlett 6i6 (2nd gen) I bought to use with my live keyboard rig and never did. I have no idea what it would cost to send to where you live and you’d need a power supply (12v, so not difficult). I’m prepared to let it go for somewhere near nothing if that’s interesting to you.

If not, no problem, but you could use a mixer as preamp to give you 4 mike inputs at once (6 if you’re prepared to put two more through an ADC first).

I have an Audient interface that expands to 10 inputs and a Roland Rubix with 4 inputs; they’ll aggregate on my Mac for 14 inputs, so I’m not leaving myself short.

 I do classical recording so 14 mikes would be one huge orchestra …