Depends on my mood, really.
Butt plugs are considered to be some of my most important tools for anal play. After a good massaging with lubed fingers to warm up the first foreign object to "go back there" is almost always my classic red Doc Johnson butt plug. It does a great job is prepping my anus for gaping (I have a big gaping fetish) and bigger toys such as dildos. I'd have to say it's my reliable standby because; all arounds it's just about perfect for me. It's easy to insert (provided I did my job in relaxing and lubing up properly), the head is bulbous enough to let me know that my ass is "occupied" and the neck holds my sphincter had the perfect half inch diameter for diliation. For the simple joy of plugging and napping while plugged, it's hard to beat. In fact, I'm "wearing" it right now as I type ;p
I also have a Pyrex glass butt plug that acts as some sort of looking glass into my rectum. It's super slippery when lubed and I LOVE to watch myself in the mirror with my ass up in the ass. Back again to my gaping thing, I'm fascinated by starring into a perfectly round black hole that was once a tight puckered ring of muscle.
The butt plugs I have with the most character are my Colt inflatable and my TOSU TWO.
First onto the Colt. I was a bit disappointed with the size upon insertion, but when I first gave the pump a good, slow, firm squeeze...WOW. If anything, this plug has helped me increase my anal capacity when it comes to girth and depth. It's so hot in thinking that "something" is inside you getting bigger and bigger. I can pump it up to roughly the size of a grape fruit. After a good pumping while in doggy styles, getting up to walk around has proven to be somewhat of a challenge because the swelling head feels like it's made my hips WIDER. I know that sounds silly, but that's what I feel.
And finally my TOSU TWO...
http://web-001.meo-team.com/catalog/product_info.php?currency=JPY&products_id=8629
Won't beat around the bush in how it works, but my god the head is BIG. Feels like I'm loading a tank shell into me. I like to put on tighty whites, assume doggy style in front of the mirror and watch my hips and butt go to work. The thrusting sensation is delicious. I like to stop midway sometimes and try to push the plug out. This makes the wide base on the plug prints through my underwear. To see a plug base THAT big wedged between my cheeks drives me crazy.