Pretty much any 040 Mac is fine. What features are you looking for? Hard requirements will make a couple machines fall off and make it clearer what kind of machine you might want to look for.
The LC475 lacks a 5.25 bay and a NuBus slot but it'll do 1152x870 at 256 colors and LCPDS ethernet cards are widely available, plus the cases are a lot less prone to some of the breaks that the 610/660 can have.
In my experience, FPU doesn't make a big difference for most 'normal' workflows. 25/33MHz/40MHz isn't something I've had time to test, but on that front, the 800, 840, and 950 each have their own go-fast strips.
Are you looking for something in particular? Like, would it be a deal-braker not to have dual displays, or do you need a particular type of disk or removable media drive?
All around, the 475, 630, 650, and 800 are great machines. 630 is faster than the 475 but has slighly lower end graphics built in (max 832x624). The 650 and 800 are both 3-nubus slot machines. The 650's got a better case, but it's slightly less pretty.
The 800 and 840 have bad case plastics. It's tough to say whether they're better or worse than a 610, but they are bad. I've seen lots of different anecdata on this. My own 840 has mostly held together fine, but I'm going to put a new PRAM battery (or just take it out altogether) and max the RAM in one go and then never pull it apart again.
I have recently handled a 950 and 9150: To be honest, their plastics are also at this point prone to being bad. What aged finely and nicely into 1996 or even 2002 really hasn't into 2019 and it's really tough to point at any single 040 68k (honestly, perhaps except the 700, 650, and 475) and go "this is the one whose case is least likely to be messed up.)
The 700 also exists and they're adorable, but you give up the 5.25 bay, a nubus slot, and it starts at 25MHz, albeit with a full '040 as stock.
Sidenote about performance: My long-term impression from back when I had a 575 and an 840 side by side is that 33 vs. 40MHz wasn't important day-to-day. I've never had a 25, 33, and 40 all sitting next to one-another, but I later had a 475 (not contemporary with any 33 or 40 system, though I could go get it and cross-bench the 475 with my 840) and found the 475 to be "fine."
This will of course vary depending on what you want to do. I'm kind of basing this on the ISO standard 68kMLAer loadout of clarisworks, oregon trail, and hypercard, which will honestly work fine on most '030 Macs as well.
The LC575 and 475 as well as some 040-based PowerBooks also exist. I had a 575 and I have a 520 now and they're great machines. The 580 is equivalent to the 630 computationally and uses a slightly lower end display. The problem is that the 500 series AIOs don't ship well.
040 PowerBooks suffer the same problems as all other vintage Mac laptops and to be honest a PowerBook G3 or iBook G3 is a better portable computer than any previous machine, primarily for physical durability and reliability reasons. The huge step forward in compute performance and things like USB, compatibility with newer programs for data migration, reliable built-in optical drives, and wifi, are mere bonuses.
So what should I keep an eye out for? 
