Too many good ones. Lets see, all the seven demon lords, grandracmon , The savers agumon Evo line, The whole gaomon line, the savers falcomon line, rosemon, Victorygreymon, vikemon, the royal kinghts.
I think that my favorite Digimon tend to be those who are partnered to my favorite human characters. In 01, I liked Gomamon best, and then in 02, my favorites were V-mon (plus Chibimon and all the various evolutions) and Wormmon.
I was also fond of Palmon and...hmm, who else...Tentomon, probably. I'll have to get back to this at a later date.
Draw your own conclusions about my favorite human characters from that.
To be honest, I'm mostly a fan of the first two seasons, so I tend not to consider Digimon from the seasons after that -- although I did think that Guilmon was pretty likable from what I saw of him.
Oh, one more! Although I don't consider Iori as one of my fave characters, Upamon was pretty darn cute.
I love Guilmon. He's cute, he's strong, and original. Original because Takato made him from scratch and because he is the only main good digimon I know who is a virus type instead of vaccine. The way he talks like an infant and generally doesn't utilize much brainpower at first is so adorable. He is playful and friendly out of battle and a fierce warrior in battle. In fact, out of every anime I have ever seen, Guilmon is one of my favorite characters. And finally, the name of his Pyro Sphere has such a nice ring to it. The name "Fire Ball" from other shows was getting really old. Fire Rocket is pretty good but Pepper Breath and Baby Flame doesn't sound as catchy (sorry Agumons).
Lets see, so many..
There's Kamemon and all of his evolutions, Lalamon too, Palmon as well. Dracmon, the PawnChessmon, Shurimon, and probably way more that I am forgetting right now
My favorite digimon, broken down by series:
Digimon Adventure: Gabumon ****
Digimon Adventure 02: Hawkmon **
Digimon Tamers: Renamon***
Digimon Frontier: Kumamon? * (haven't seen much of the series)
Digimon Savers: Gaomon ***** (winner)