Out of the three you mentioned, I'd say:
Necromancers are good damage dealers/debuffers and can add protection and support to the party through minions and condition removal. They have the best energy management in the game (including paragons). Necromancers are versatile, as they can specialize in minion mastery, AoE damage through curses, party support and spiking through blood, and so on. They make good farmers, as well.
Elementalists are very strong damage dealers, pretty decent debuffers/snarers, and may add party support through wards. They have three of the best E-management elites (Mind Blast, Elemental Attunement, for use with a normal attunement, and the newly buffed Ether Renewal), but are otherwise seldom good at managing their large energy pools. Elementalists are fairly versatile, as they can make powerful AoE nukers/DoT, snarer/debuffers, air spikers (and addparty protection with Blinding Surge), Warders (with strong earth damage spells as well), and so on. They make good farmers too.
Rangers are decent ranged single-target damage dealers, have pretty nice AoE with Barrage/Volley, have traps that inflict damage and impose variosus conditions, but excel at spreading conditions (poisoned, crippled, bleeding, blinded, and DAZED) at range and interrupting (see also, DAZED). Rangers have pets, which can add a rather nice ammount of damage to the repertoire. Rangers, in contrast to the casters mentioned above, survive. They have the highest unmodified armor agianst elemental damage in the game, and have decent base armor as well, combined with a large pool of stances for blocking and kiting (although the latter applies more or less only to PvP). They have good energy management through Expertise (and, if needed, elites such as Prepared Shot). Rangers are extremely versatile, as they can spike, deal AoE damage, tank, trap, spread conditions, interrupt, run for their lives, and so on. Rangers can farm with moderate success (Forest of the Wailing Lord works for farming shards, glacial stones can be farmed with touch ranger, etc.), but are often sub-par farmers than necromancers or elementalists (and monks, and sins, and rits, and wars, and dervs...).
My primary is a ranger.