First commonly the best is Nordenwatch because there usually is a tug of war of push or be pushed. Khaines has exploding things that make a tug of war hard to do and Gates of Elkrund is good but plenty of los, levels(Warhammer Online Gold) and entrances to not make a solid deathmatch. Norden is usually preferable because the warcamp has the closest trainer so you can pop out turn in scenario and kill quests, get trained, and pop back in. Leveling is quick in T1 so really any of them is good but continually turning in the SC quest helps alot too and running around to turn them all in can restrain leveling.
Secondly mourkain Temple. Objective grab the flag kill the enemy. We used to hate how often MT would pop (warhammer gold)until we played a character with sole intention to level(Warhammer powerleveling) and then hated when any scenario other than MT popped. MT almost guarantees a 500 game, even the close games are 500-490 but hardly ever does a game get to 15 minutes. PT and ST will have more 15 min games and less 500 games because they are objective based, reqire more running around.
In PT it is wise to defend the flag but if you are defending you are not gaining xp until the enemy comes closer, which can be forever in a turtle match. ST almost the same, a lot of time spent chasing after the fc then actually fighting. In PT and ST teams can easily get spread out where in MT there is one focal point.
At last doomfist Crater, Tor Anroc, Temple of Isha. The way to get the most xp out of a scenario is to constantly be around the activity and for you to die less and kill more in Warhammer. For a scenario, the most rewarding is a scenario that basically puts your team up against the other team in a straight up deathmatch(warhammer online gold). Basically scenarios that involve less running around and have higher chances of getting to 500 under 15 minutes.