Scyther is in theory the best one there for battling.
Butterfree-Unlike most people, my favorite Butterfree is a sunnybeaming Butterfree. And now that Butterfree get Morning Sun, sunnybeaming Butterfree has just got even better.
Parasect-He has slightly below average stats (except, perhaps, attack) and his speed is really a let down. Parasect being one of the first Aromatherapers (before Blissey and everyone else took over) and being able to use Spore makes people use him once in a while. His stats are in the region of the 'unevolved Pokemon's' and his defenses are so low that noone really uses him anymore though.
Scyther-A great baton passer and physical sweeper. Scyther and Venomoth are just about the only ones you will ever see on Netbattle. A great attacker, but a bit lacking in defenses. Scyther does die from Rock Blast though. The Hoenn base 600s always beat it too. Zapdos sort of beats Scyther due to the types. The Kanto physical rock couple and Steelix will beat it too because they have physical attacks to high for Scyther and Golem and Rhydon get stab on rock attacks and they do not usually faint from physical attacks (even super effective ones). Skarmory will beat Scyther just like it beats any other physical sweeper (except maybe the ones which can Focus Punch it when the opponenet switches out the Blissey like a CB Slaking or a Medicham). There is also no way that Scyther will be beating CM Suicune either. Scyther is the fastest UU Pokemon that gets Reversal which is nice! Just watch out for Golem.
Venomoth-Pretty useless and too slow to be a sunnybeamer imo. The only useful things it can do are SIgnal beam and Slugde Bomb.
I have never used Ariados so I guess I cannot write my opinion.
I voted for Scyther as he has the best movepool. He is a good Parasect, Breloom, Heracross and LB Nincada counter.