Black Marsh and Morrowind (without Vvardenfell, maybe?)
Elsweyr and Valenwood (and potentially Alinor)
High Rock and Hammerfell (maybe main quest takes you to Yokuda)
Or come totally out of left field with Akavir
As a dedicated Argonian player, I feel that the honest best course of action for roleplay puropses is specific to the particular character and boils down to his or her stance on two issues:
ISSUE 1: RACISM
Racism, or at least inherent racial bias, is endemic not so much to the Stormcloaks specifically as to the Nords on the whole. Indeed, Ralof and Hadvar are some of the only Nords on either side never to call the Dragonborn a "lizard" or some similar slur (Galmar does it, the Imperial officer calling you up to be executed does it, and so on). Ulfric, despite his despicable racial policies towards Argonians and Dunmer, seems to hold no personal bias against the Dragonborn, and it appears that the rest of the Stormcloaks are willing to make a personal exception for the Dragonborn after he or she proves him- or herself with service to the cause. Should the Dragonborn's principles be racially defined, that is, "any slight against an Argonian is a slight against me", then the Stormcloaks' attitude, and the overall Nord attitude, are distasteful to say the least, but, for a more selfish character, shouldn't matter as much. On the whole, Imperial soldiers and officers are far more racially diverse than Stormcloaks and seem far more tolerant of the Dragonborn, but it should be kept in mind that they are, at least for the moment, acting on behalf of the Thalmor, who are the objective bad guys and obviously have no good intentions for the Argonian race. In addition, isolated examples of bigotry continue to exist within the Imperial fold even while the Dragonborn is making a name for him- or herself within it.
ISSUE 2: THE THALMOR/THE FUTURE OF SKYRIM/THE EMPIRE
This is complicated, to say the least. The Stormcloak revolt is predicated exclusively by the actions of the Aldmeri Dominion: the ban of Talos worship, the openly stated plans to end all races of Men, the overarching presence of an Elven authority in Skyrim for the first time since Ysgramor kicked out the Aldmer in the Merethic Era. None of these issues apply particularly to Argonians, but dissatisfaction with the Empire and desire for self-determination are nothing new to them. Indeed, the events through which Black Marsh broke free from the Empire in the early Fourth Era are not entirely dissimilar to the Stormcloak rebellion. However, without a powerful Empire of Tamriel, nothing protects the fledgling Black Marsh/Morrowind Argonian-ruled state from an all-out Dominion invasion, which would likely result in widespread destruction and loss of life among both Argonians and Dunmer (the two races have their difficulties, but I like to think that their diasporas in Skyrim have some degree of solidarity), which, regardless of the Dragonborn's stance on the validity of Ulfric's claims for rebellion, are definitely not desirable to him or her as an Argonian. It's possible that the Argonian state, through guerilla tactics and the skillful use of the unforgiving terrain of Black Marsh and Morrowind, could repel such an invasion eventually (just as the Redguards did in Hammerfell after the signing of the Concordat), but this is not a certainty and the significant loss of Argonian lives would be all but assured. In addition, the Empire of Tamriel is a significantly more favorable environment for Argonians than the Aldmeri Dominion, and dooming it to eventual conquest by the Thalmor would not be desirable. By stemming the rebellion and killing Ulfric, the Dragonborn may be going against his or her principles, but leaving the Empire strong enough to revitalize itself and stave off the Dominion in the south, perhaps with help from a reconciled Argonian state and a reintegrated Hammerfell.
Overall, this kind of reduces itself to "do you, as the Argonian Dragonborn, sacrifice your principles of self-determination for your survival with the Empire, or potentially doom all of Tamriel to conquest by the Thalmor, but stick with your principles and side with the Stormcloaks?" Racism is a notable issue, and Ulfric's personal behavior towards the Dragonborn seems incongruous with his oppressive policies in Windhelm, but many on the Imperial side are just as bigoted and the Aldmeri Dominion likely views the Argonians as just as, or even more, undesirable than Men.
Just some food for thought.
(I may be hard-pressed to update soon. Just a heads up)
Character Update: Kavatha
Race: Argonian
Level: 4
Skills: Destruction (28), Restoration (30), Illusion (20), One-Handed (23), Sneak (20)
Kavatha ascended the staircase to find a dragon sitting atop a strange wall. (Keep in mind Kavatha hasn't discovered Fus yet). After a furious fight and no shortage of Healing on Kavatha's part, the dragon fell to an Ice Spike. After absorbing the soul, Kavatha looted a chest (89 Gold, Adept Robes of Restoration) and approached the wall. He felt a power empty into him. Feeling the need to speak, he whispered Laas. Life. Suddenly all beings in the area became known to Kavatha.
(it's all done). Kavatha finished his house. He went to Shor's Stone for supplies, but then noticed a dragon roost on the mountain next to it.
(Just enough for an entrance hall)
He was almost broke, but Kavatha had succeeded in gathering enough supplies for a basic home on the coast. He bagan construction.
Kavatha, after some haggling, bought enough iron for his house.
(Oh. It was gonna be a nice coastal villa).
Kavatha continued back to Shor's Stone. He greeted the miners. Kavatha decided to build a home on a riverbank close to Shor's Stone. (House Coordinates: K-7) He went to the nearby city of Riften to buy supplies.
Kavatha rushed over to the dragon that had knocked Harold over. He thrust his sword into the dragon's head, killing it. Some sort of mystical light began to drain into him.
Kavatha, on his travels, had found a Spell Tome of Ice Spike. Now was the best time, he decided, to put it to use. Kavatha shot two Ice Spikes at the dragon, striking it on the wing and neck.
"I felt a strange attraction to the place." Kavatha walked over to Maracon and greeted him, having met him at the College.
"My name is Kavatha. Pleased to meet you." Kavatha looked around. "Mind telling me what this place is?" Noticing the Deathlord, Kavatha bent down. He picked up the Draugr's Ancient Nord Sword of Chills.
Kavatha finnaly arrived at the barrow. He walked through the dungeon, noticing corpses strewn about. He walked into a main antechamber and saw fighters engaged with what appeared to be a giant zombie! Kavatha thought quickly and showered the monster in Flames.
Kavatha killed the spider after a huge fight. He rescued the miners, who thanked him.