All teams have their good and bad points.
The skaven are fast but quite lightly armoured and so can take a bit of a beating, if you can catch them.
Orks are not particularly skilled but are tough and will take and hand out a fair bit of punishment.
Humans are generally the middle ground
Dark / Elves are fast and agile, also lightly armoured but are expensive in points so you don\'t get many team members for your opening squad.
Goblins are fast but not agile nor well armoured... but they are cheap as chips so you get a lot for your starting team.
Undead are not well skilled or agile but are damn hard to kill so what you pick for your opening squad is likely to stay in the squad for a long time picking up new skills and such. Allowing you to spend your funds on star players or the more expensive team members.
Dwarfs are slow but fairly well skilled and tough.
Your choice of team really comes down to personal choice and whether you will be playing one off games or a longer campaign/league. Undead are not too good for one off games for the reasons above, but over time they become a force to be reckoned with. Goblin teams can be beaten down quite quickly but then they can be fun to play.
Humans and orks are probably the best for one off games - after all that is what comes in the box