If you DO happen to find copies of the complete collection, do share with us xD haven't seemed to find one available for quite some time now.
As for campaigns, if you want to start like most of us did then Prophecies is your best bet. It has all 6 core professions- Warrior, Elementalist, Mesmer, Monk, Necromancer, and Ranger, and has quite a nice introduction in Pre-Searing Ascalon. It will definitely be the longest in terms of time it takes to complete the whole storyline, but it's very lore heavy, so if you're in to lore, I'd highly suggest Prophecies.
Factions has the same six professions plus two more professions unique to it- Assassin and Ritualist. Assassins are pretty fun to play and are involved in just about every single end-game farm/team clear there is, so it's a very lenient profession in terms of playability. Ritualist is more backline oriented in it's damage/utility/healing roles as it relies on the use of spirit spawning, but it's also a very rewarding profession and will definitely be unlike any other game class you can play.
Nightfall is a rather interesting one, as the storyline could have been shorter IMHO. It's actually the easiest in terms of solo play as you unlock heroes as you play, which are basically henchmen with customizable gear slots and skill bars. It also has 2 unique professions along with the 6 core ones- Dervish and Paragon. Dervish wield scythes, which, if enemies are grouped correctly, will hit all enemies in a small cone in front of your character. It's an easier version of the Warrior profession IMO. Paragons wield spears and are a sort of ranged utility profession, in which they can both do damage and buff party members with shouts/chants. You don't see a lot of these anymore since their damage is rather low compared to the 9 other professions, but are still fun to play nonetheless.
Eye of the North you cannot play unless you have one of the other 3 expansions purchased on your account, so I'd recommend one of the other 3 first
tl;dr- If you like lore, play Prophecies, if you want to get through the game quick, play Factions, if you want to make it easier for yourself, play Nightfall. Like cosy said, the trilogy is actually quite cheap if you buy that instead of purchasing the 3 separately.