Soloing to the 45~50 range is the easiest, because you can easily find orange/red/purple mobs that don't hit hard enough to matter and have low(ish) HP that go down fast with FB. After you get too high of a level to get good EXP in CL3, things get harder. Here's a rundown of where you can grind:
-EC: EC's first mobs turn purple at 41, but most are purple by 43 or 44 I believe. There's not much color difference here despite the huge size. Most mobs will be green at 51, but some may still be yellow. That may seem bad if you expected to grind on stronger mobs, but their HP is lower than mobs in Pando, the Jungle, and DD1/DD2. You'll kill them fast-ish, but they won't provide much EXP.
-Pando: Pando has a much greater level range than EC. The mobs start turning purple at 41, but some don't turn purple until 48 or 49. Whether or not you can solo here really depends on your gear. You'll probably want a water top or the water ele skin buff, and try cali/viri. If you can't tank there yet, drop back to vails or 'firemobs' (the bears/monkeys/landers and the link near cali). You *should* be able to tank here at 51, regardless of your gear, and the mobs will be orange. Good EXP, but VERY slow kills. You can stay in cali and viri until around level 54 for good EXP.
-Jungle(Dino)/DD1: These are harder to tank, more crowded (generally), have very few good places for pulls ("mixed" in the Jungle is by far the best -- that's the little valley west of the path about halfway up on the light side), and you're highly likely to be attacked by the opposing faction. It's more or less preference, and it really depends on how long you can stay there without being killed by fury or if mixed is free. (At mixed, you can do pulls of 6 mobs [with one pull having 7] and they should be respawned when you get back. Everywhere else in both the Jungle and DD1, you mostly find pulls of 4 with slow respawns.)
-DD2: This will be the best place to grind after you hit 54 or 55. It's usually completely free as noone goes there anymore. It's great EXP, great pulls, but somewhat hard to tank and the mobs have quite a bit of HP.
-VR: VR turns purple at 51, but the mobs here are significantly harder than anywhere else. You probably won't be able to tank it until you're level 57 (when they turn yellow), and that's with good gear. If you can't solo there at 57, and you still REALLY want to solo, go back to DD2. But honestly? Most people will have no problem with a REC priest healing them in VR. Priests are sometimes hard to come by.