Tough to pic an overall favorite. Also different paint schemes look better on certain airplanes, but here are some favorites:
Gulf Air L1011 "Golden Falcon"
TWA late 90s-2000s livery with globe on forward fuselage 717, 727, 747, 757.
United SUD-210 Caravelle
Canadian 747-400 "Proud Wings"
EVA 777-300 in special "777-300ER" colors
Cargolux 747-8F with red tail
Boeing Dreamliner livery on 737 (Alaska), 747-400 (China Air Lines), and 787. The Red/orange dreamliner livery on the 747-8i especially stunning.
Singapore 747 in standard livery & Tropical livery, Concorde
Cathay Pacific "Spirit of Hong Kong" on 747-200 and variation on 747-400
British Airways: the many "World Tails", Concorde in the livery with blue on top half of tail and red cheatline running down fuselage above pax windows.
Thai Airways International MD-11 old livery
Virgin Atlantic 747-400 and A340 livery where the fuselage looks silver and has a broad blue border forward and under the red on the tail.
Korean Air lines A380
Air India 747-400 old colors
Aeroflot Il-96 new colors (silver)
It is much easier to answer the question of ugliest paint scheme: Any aircraft from any airline where the fuselage is all white with the only color being on the tail...I've heard them called "White Whales" Billboard titles are also very ugly.