The FlyFIve33 TinyTrainer is the most fun quad I've ever built.

Part of this has to do with the alternative powertrains I've come up with, part of it has to do with the raw joy of flogging a deliberately overweight 3" quad for all its worth.
Here's what I came up with when trying to build a budget TinyTrainer that can use $4 CNHL batteries (2S 650mAh packs).

Frame - the TinyTrainer, intended to visually be a compact version of the DRL Racer is obviously a key component. The unibody frame originally concerned me for durability, but I've destroyed one of every other 3" frame in the time I've had these built, and these three quads have seen more packs through them, and more crashes, than the other 10 quads (in 3") that I've built put together. It's an incredibly solid frame, especially given the weight.
Motors - I went with budget motors, which happened to be 1106 6000KV setups and the RCINPower 1207 6000KV motors - all of which I've been able to find for $10 or less at various sales. So far the favorite are the 1207 6000KV options, because they are much simpler to tune, and provide a bit more raw thrust, but are efficient enough on 2S that I can get similar flight times and performance to the legit 1404 3S setup these are intended to be ran on. Practically, it's hard to argue against the TinyTurner 1404 motors on triblades for feel, but I feel like this alternative powertrain actually has worked extremely well for my intended uses, namely running a mountain of cheap batteries shared with our SouthWest Pod Racing SPEC class (2S 650mAh on GTB-339's), so I can use the same batteries.
Practically, the 3S packs still work actually too well - even with throttle limiting, improving battery performance results in comically fast setups that perform amazingly well, but burn down batteries in 90s.
Another adjustment I've made is props - the 1404's feel fantastic on the GF 3016 triblades, but these taller motors actually see similar feel running the 3018 biblades, and I actually put down slightly more consistent times... the good news is that there aren't bad options for propellers with these, I'm always partial to Gemfan props.
AIO - The JHEMCU 25A AIO Pro is what I recommend now - these don't need the extra amperage, and the features all work. It's a compact build, but works, these have stood up to great punishment. The GEPRC 20A and older JHEMCU units have worked extremely well for me. As always, I recommend JESC 2.3 on 48kHz with startup power set to 0.25 and timing on MedHigh, and you can use my Betaflight Community Preset for 3" quads on these (with a bit more gains, but to start off, that works). As always, I highly recommend adding a 16V 220uF capacitor to these, it helps.
FPV - Small budget parts carry the day here - the Caddx ANT nano fits really well (As does the Runcam Nano2 camera), and practically every other Nano camera I've tried doesn't fit as well, and costs more. The VTX is a PandaRC 16x16 Nano unit, which is perfect for this frame - just put it on a shared standoff with the AIO board and let the canopy on the quad hold it in place, which can also be VHB reinforced with a capacitor. There is also a great place to mount a nano sized u.FL antenna, just wrapped around (see picture) with the antennas all hanging out in the back. The XM+ is plenty good for this, just run RSSI firmware, and I can make it work.
LEDs Tiny's LEDs makes a dedicated kit - the Whoop PodLite is awesome, and I was able to cobble together my own super-budget set of LEDs to run on the arms from that - end look is really quite good, but practically if your time is worth more than $5/hour, just buy the full kit that you want, it's highly recommended. I do wish that Fly533 would carry the Tiny's Kit, and/or Tiny's would carry the TinyTrainer.
Batteries - This was actually the secret motivator for this build. $4 batteries are too hard for a cheapskate like me to pass up, and the 650mAh packs can actually let these fly for five and a half minutes... or just stay absolutely wide open and be back at storage charge at around 1:38-1:41. That seems like a short flight time, but I have been messing with setting the throttle to run on a switch and fly with just the other three sticks, and this setup lets me get away with those shenanigans... and it isn't as though I'd feel bad about hurting these batteries, but they don't seem to care.
There is a battery mounting modification required - either print out the 650mAh sized battery sling, or like I did, just zip tie a 135mm battery strap onto the bottom and call it good. Beyond that, I feel like these are an even better battery option if the intent of these is just for fun... otherwise go full Spec Racer with those 1404's that perform better.

These are the most fun quads I have, and actually see the most use.

I have way too many quadcopters to choose from, but because I can charge up my fleet of cheap 2S packs and just go have fun flying nonstop for an hour, these get the nod. I usually brind other quads along, but the box with these quads and batteries always come along when I go out to fly.