I recommend any of the ones I know:
There are two current runs which I think are good as starting points:
- Teenage Mutant Ninja Turtles: Saturday Morning Adventures - It started as a mini-series but it's ongoing now. It's a continuation of the 1987 animated series, so it's in the same style (goofy, appropiate for kids, still fun for adults).
- TMNT ongoing by Jason Aaron. I'm not personally a big fan of his writing, but I'm still reading this one and it has some good ideas. Most people love it, I think it's ok (the art is awesome though) so I'm an exception here.
You can also go to where it all began with the original Mirage run. It was created by Kevin Eastman and Peter Laird. I think it's really good and has aged well. There are collections by IDW like the TMNT Compendiums, which collect most of them in 3 volumes. They're massive books, but I'm sure they're easier to get digital.
There's also the IDW 2011 reboot which is great. I've only read some of the first 40 issues I think. I will definitely go back to it, but the collections publication was a bit weird when I started reading it. The current ongoing is a new volume following this run (but I don't think you need to know much from the previous run to enjoy it).
Finally, you can also check out The Last Ronin. It's written by Kevin Eastman and Tom Waltz (who also wrote the 2011 IDW run), but it's based on an idea Eastman and Laird had back when they were writing the Mirage comics. It's based on a future where 3 of the 4 turtles are dead and the one left wants to avenge his brothers. It's dark and gritty, like a "Dark Knight Returns" of the turtles. It did so well that they kept expanding that universe with more comics.
I will also mention there's a Compendium of the Archie TMNT comics that was published recently. These are the comics I read when I was younger and they're really good. It started out based on the animated series from 87, but it became its own thing and moved to doing original stories, characters, etc. I really like this one too, but it might be nostalgia ๐ฌ
If this is too much information, I would recommend pick any of:
- the current Jason Aaron run
- the IDW 2011 reboot or
- the original Mirage comics
And see what you think.