Simple answer:
Get to a Mesa dealer and demo, demo, demo in a quiet room until your fingernails are worn down. Mesas can be tricky to dial in, especially if you're not familiar with them, so read up on each amp before you plug in. A bad set-up can really undermine your exploration! You'll know when you find one that speaks to you, one that feels comfortable and right.
You'll get plenty of opinions here, but only you know what sounds and feels right to you. Youtube videos will give you some sense of each amps sonic character, but the sound quality isn't very good, nor will it necessarily translate into playability for you. When I'm feeling "restless" with my tone, I'll check the Guitar Center used gear for something interesting. It's a great comfort knowing you can demo the amp for a month, playing with YOUR gear, in your applications, and return it if not pleased. Patience yields big dividends when looking for a tube amp that meets your needs...
I don't know how familiar you are with tube amps, but they can be double-edged swords. They can reward you with moments of pure bliss, and the tone and feel can be easily tweaked with different tubes. They can also be finicky and temperamental as they employ early 20th century vacuum tube technologies. Point being, be prepared to read and learn a lot about them if you want to "maximize" your tone quest. This board is populated by some very VERY knowledgeable people who have decades of experience with tube amps in general, and Mesas in particular. Don't be afraid to use the Search feature as the archives are rich with information and opinions. Good luck and welcome!
Personal preference/Not-so-simple answer:
I've owned several different Mesas and demo'd many, but I'm a Mark series guy at heart, owning a "Holy Grail" Mark IIC+. But, I just snagged a DC-3 for $525, never having played a DC before, and I'm totally blown away. Honestly, this amp recreates in my ears the sounds in my head, and inspires me to go further. It has a very natural feel and is pretty forgiving...but that's just me. The Dual Calibers are forgotten gems from the 90s. With two truly independent channels and a graphic eq, it's hard to find a Mesa with a broader tone palette, especially for the price. It's all there: country, jazz, rock-n-roll, hard and classic rock, metal...and it's pretty easy to dial in, which is important for newbies. They come in two basic flavors: EL84 (20 or 35 watts) or 6L6 based (50 or 100 watts).
The rhythm channel goes from crystal clean to Black Face vintage clean to a nice post-early break-up bluesy sound. The Gain control is very useable, as is the Presence control. The Lead channel can get raucous very quickly (above 3), and may be too saturated and compressed for some, but it's easily tamed with proper preamp tube selections. If you are so inclined, the tube assigments make it easy to "roll" preamp tubes: influence the gain and tone of the separate channels with different style preamp tubes. It might not have the laser focus of a Mark, but it covers a lot of ground very well and can get very heavy. They enjoy Mesa durability and they are bloody bargains!