Try Smooth Experience Bar if you want quest XP, rested XP and session stats on one bar. If you only want to move or resize the default bar, Movable Menu Frames does that and adds nothing else.

Add-on What it adds Good if
Smooth Experience Bar Quest and rested XP, session stats and themes You want to see your levelling pace
XPBar Island Experience, reputation, honour, pet XP and skills in one place You follow several things at once
Movable Menu Frames Moves and resizes the default XP bar, bag buttons and micromenu You like the default bar
Pet XP Bar Plus A small XP bar and level readout for a Hunter's pet You're a Hunter

Smooth Experience Bar's author modelled it on the Luxthos Experience Bar, a WeakAura. WeakAuras itself has no Forever build, so an add-on like this is the way to get that look.

We list these from each author's page and haven't tested each one on the live client. Each page shows the date we checked.

Levelling faster

An XP bar shows your pace, but it won't improve it. See our levelling add-on setup and the levelling routes on our sister site.