I have decided to break down and buy a Glock. While I do like the 17 and 34 I have decided on a big bore model. However I cannot decide on the 45 or the 10mm. Other than 45 ammo being cheaper, what are some major factors in choosing in between these two calibers...And recoil is not a factor either, big bore revolver guy here.
Getting back to the OP's question, the first Glock I found that I could shoot well was a 30. I immediately bought one, and it's both my primary CCW and the most fun to shoot. I added a 34 for a 9mm range gun, and then a 21 because I enjoy the 30 so much. The 21 was the least satisfying to shoot, until I changed the trigger connector and spring to a Ghost 3.5. Now I enjoy shooting all three.
If I could only have one, the 30 would be it. It's an amazing "small" .45, far more accurate than it should be, and surprisingly mild on the recoil. By the time I'm putting the 5th mag in, I'm fighting the urge to laugh - it's just a hoot to shoot. It's completely stock - I prefer my carry guns that way.
The G34 is also stock, but it's a "competition" setup in factory configuration. It has a lighter trigger, longer slide (lightened), and adjustable sights. Once I learned a grip that works for me, I shoot it very well. Ammo is cheap, and recoil is very mild.
The G21 is the full-sized version of the G30. Here in CA, all three guns use 10-rd mags, so effective capacity is the same. That destined the 21 to be a range pistol for me - the 30 is nearly as accurate, more fun, and holds as much ammo. The factory trigger was what you would expect in a service-model Glock. Simple and inexpensive to change, and I'd actually bought the Ghost kit for the 30 when I bought it - then after shooting it, decided it was perfect for a carry gun as is. The kit was sitting there, and I wasn't happy with the 21's trigger...20 minutes later, it was all good!
If choosing 21 vs 20, I'd go 21. You can always get a conversion barrel and a couple mags and have both. Can't do that with the 20.