I own (and use quite frequently) the following circle punches:
3/4" - Stampin Up
1" - EK Success
1 1/4" - Stampin Up
Here is what these punches look like when punched out:
Quite possibly the most used punch that I own: Stampin Up 1 3/4" Scallop Circle Punch: newest Stampin Up Scallop Punch which is much larger - so I can't tell you much about that. I don't really do much with square punches (isn't that odd? LOL!) I do own a 1 3/8" square punch. But I hardly use it. Other favorite shaped punches: Word Window Punch: there's lots you can do with this one. It's great for stamping or punching out printed greetings onto a small strip...
or even to create an opening to slide a gift card or small treat into (in this photo, the Kit Kat bars slide into the holder via a window that was punched using this punch)
Stampin Up Round Tab Punch: this is great for adding greetings & also in scrapbooking you can add captions to photos. You can also use it for other things like the topper of a Christmas Tree Ornament, or to join 2 circles together to make a circle card: (NOTE: in this pic below, you can see that I nested a 1 1/4" circle onto a 1 3/8" circle onto the SU! Scallop Punch. The Reindeer stamp is from "Deer Friends" stamp set from SU! and fits *perfectly* into a 1 1/4" circle punch)
Oval Punches: I own a few Oval Punches through Stampin Up which I also think are great to have. They don't really nest that nicely, but they are still fun to use together: 2" x 1 3/8" Oval Punch and 1 3/4" x 7/8" Oval Punch. I used them in this project to create these mittens (I stole this mitten idea from my SU! Demo Michelle. If you are looking for a SU! Demo to order through I highly recommend her! She does long distance clubs I believe too!)
Border Punches: I love me some border punches. They are such fun. I suppose I have a slight obsession with ones that are scallop-like in shape, no?
The ones pictured above are from (left to right):
Fiskars Apron Lace Punch
Stampin Up Scallop Edge Punch
Stampin Up Scallop Trim Border Punch
Fiskars Threading Water Border Punch
I own many more border punches. Mostly all EK Success, Fiskars, Martha Stewart. They are all great brands.
My recommendation for most stampers would be to build your circle punch collection. I can honestly say that I consistently use most all of my circles. (there are many stamps designed to fit inside circles - so it helps to have various sizes on hand so that you can make it work to whatever stamp you're using) Look for bargains (Joann.com usually has great sales & I believe they carry the EK Success Paper Shapers).
As for larger circles - I don't have any punches larger than 2". Once you get up over 2" the punch becomes somewhat cumbersome to use. At that point I will either use a circle cutter or my Cricut/Silhouette to cut the size out that I want.
I hope this post answers some of your questions. If I have let anything out, please leave me a comment & I will try to get back to you as soon as I can. Thanks!