“I don’t always like advertising spokespeople, but when I do, I drink Dos Equis”
I always liked Erin Esurance, may she rest in peace.
And “Pat” the dumbassed Toyota fanatic. He’s so earnestly clueless.
I enjoy the Geico gecko, mostly.
And JK Simmons as the Farmers University guy.
I’ll never use any of these products (I’ve had Allstate since I learned to drive. Oh, and I like Mayhem and Dennis Haysbert, too.) Instead, I view the commercials as a TV series with very short episodes.
“Super Creepy Rob Lowe” needs his own TV show, like, now.
I thought about complaining that Lowe’s an endorser, not a character, and the various bad versions of him are not continuing characters, and so don’t count. But I’m not gonna, because I love those commercials. As someone wrote upthread, those commercial are basically very short TV shows–worth watching for themselves. I look forward to each new iterations.
Sadly, that does make sense. But at least she’s winning fans. That could pay off in more lucrative work, down the line.
Hopefully she was wise enough not to sign a contract when the campaign began, giving her more leverage later on. I guess I’m hoping she’s a Don Draper fan–a very odd thing for me to write. 
He has sometimes given good advice, without a doubt.
Those weird rat things (spongmonkeys?) from the old Quiznos ads. Only thing that ever got me to go to Quiznos. Then the food made me turn around and leave again.
I was going to mention the Dos Equis guy, but Wilson beat me to it. Another was the Miller High Life delivery man; he died young a couple of months ago. 
It jumped the shark hard, but the first few adverts for Compare the [del]Market[/del] Meerkat featuring Aleksander were very clever.
Flat Eric - filthy beats and a chilled puppet.
Another vote for Mayhem. Love him!
Also, the “original” Old Spice guy (“the man your man could smell like”). I think I’ve watched his original commercial like 500 times.
Well, since my first two were taken, I’ll have to nominate…
Dang it! Apparently I’m incapable of an original thought tonight.
Mildly hot? I ought to dunk your head in a pot of boiling water.
I don’t have any names that haven’t already been mentioned, but since the OP disallowed pitch(wo)men, my current/most recent Top 5 would be Lily (AT&T), The Most Interesting Man in the World (Dos Equis), The Man Your Man Could Smell Like (Old Spice), Flo (Progressive) and Mayhem (Allstate).
Has everyone forgotten Jim Varney as Ernest P. Worrell? Quite a career off of one meatball character. KnoWhutImean?
The “Pizza Hut Girl” from the Pizza Hut/KFC/Taco Bell Star Wars tie-in ad campaign.
An elegant character for a more civilized age…with a good blaster at her side.
Along the same lines, the unnamed woman from the animated Rally’s/Checkers commercials. Designed by Peter Chung, so it’s a bit like “Aeon Flux goes Rockabilly, plugging Wendy’s.”
Mayhem (He dates Carrie in an episode of Sex in the City) and Joe Isuzu. Joe Isuzu completely caricatures ALL salemen, especially car salesmen.
I always liked the really deadpan guy who interviews a bunch of pre-schoolers around a tiny table for (I think) a cellular company.
I guess his series of ads has been retired since I don’t see them anymore.
Mayhem is played by Dean Winters, formerly of LAW &ORDER: LET’S FETISHIZE RAPE WHILE PRETENDING TO BE ALL RIGHTEOUS AND STUFF.