So, here are my favourite buses in terms of design and attractiveness!
SINGLE DECKER
1.
Mercedes-Benz Citaro

Amazing looking bus, very nice and well designed! Fantastic interior too! I like all three version of it, but personally, the Citaro C2 (the one in the photo above) aka the current Euro 6 version has got to be the best. It really is amazing!
2.
Scania OmniCity
Very nice looking too, especially the Euro 4/5 versions! It looks awesome, and like the Citaro, it also has an awesome interior!
3.
IIA / Breda MenariniBus Citymood / Avancity / Vivacity

(Citymood)

(Vivacity / Avancity - They look exactly the same but I'll explain the differences in the next post)
Italian buses may be very unreliable, but they can at least design nice buses! And the Citymood / Avancity / Vivacity prove it! And the interiors are also amazing!
4.
Solaris Urbino III / IV
(sorry no photo available)
Attractive and sleek design. The III especially, I love the slanted curve towards the door at the front of the bus!
5.
ADL Enviro200 MMC / 200 (old style)

(Enviro200 MMC)

(Enviro200 old style)
My favourite out of all British built single decker buses. Especially the Enviro200 MMC, it looks really attractive and amazing! And once again, the interiors are great too!
DOUBLE DECKER
1.
Scania OmniCity DD
My favourite ever double decker bus. Like with the single decker OmniCity, both its exterior and its interior look so nice that they'll encourage more people out of the cars to use buses!
(P.S. how funny that the photo I sent to you is my profile picture - after all that bus works one of my local bus routes whenever I'm in the UK)
2.
Wright Gemini 2
My favourite out of all the double deckers built by Wrightbus. It really looks modern and sleek!
3.
ADL Enviro400 / 500 MMC (they look the same outside) / City / 400 (old style)
(photos will be sent in my second next post tomorrow due to Railforums not allowing more than 10 photos in a single post for some reason)
Amazing and sleek. One of the best ever British built double decker buses when they come to attractiveness. Especially the Enviro400 City, it looks absolutely stunning from the outside! And inside too!
4.
MCV EvoSeti

Nice, sleek and modern! Very nice modern Euro 6 double decker bus!
5.
MAN Lion's City DD
(photos will be sent in my second next post tomorrow due to Railforums not allowing more than 10 photos in a single post for some reason)
Well, just like with every other bus I sent in this post - it looks really nice and attractive!
Now for the differences between the Avancity and Vivacity. Whilst they look exactly the same both outside and inside, the differences are that the Vivacity is the midi-bus version (available in 8m and 9m versions, with two or three [9m only] doors). The Avancity, on the other hand, is the full sized single decker bus version (available in 10m, 12m and 18m bendy bus versions, with two or three doors). So effectivelly just think about how long the bus is in order to get the difference. And btw, the Citymood is the successor to the Avancity.
Oh and that if the bus of that design has three double-leaf doors, then it's definetly going to be an Avancity. This is because in the three doored Vivacity, the third door (the one at the back of the bus) is single leaf, due to the Vivacity being a midibus. If you see a single leaf rear door on a 3 doored bus of that design, then it will be a Vivacity. (EDIT: this was originally in a seperate post, but because it got deleted for some reason, I reposted it here)