What is the best place to level up?

Try Dimensional Vortex (Antiquity) - Frozen Cliffs. Near the end are 2 Snowbeasts (dark blue Heckran palette-swap with an *Ice II, *Water II, Slap, Repeat routine and power boosts when attacked). It may take a while to get there, but the experience is worth it (plus the battles on the way and the few optional Snowbeast fights on the previous screen).

You may need to use all of your strongest Magics and Techs to win, but do NOT use water based attacks (they are immune or absorb it, I forget which). *Flare, *Luminaire, *Dark Matter, Electrocute, Antipode Bomb III, Delta Storm, etc. work, just no water based damages. The last Frozen Cliffs screen (the one with the exit to the cave leading to the Shade) is small and the Snowbeasts are quite easy to initiate a battle with (a forced fight).

I used Dimensional Vortex in 1000 AD. The last room before the Twilight Grotto. The red ghaj's give great Exp.

I know exactly what you mean, I found this really great spot in the black omen

The room where you fight a blubber hulk and two flyclops in one battle and then a cybot and one of those sentinel lazer things. (fourth room after the nu shop room)

What I did was I had crono and magus in the party and gave them each a golden stud accessory (if you don't have very many, you can use charm or twin charm on a flyclops and they give you one) in the cybot battle have crono or magus use luminaire or dark matter to finish them off in one blow, then for the blubber hulk battle have crono use luminaire (that should kill blubber hulk) and then have magus finish off the two flyclops with dark matter (or any other spell for that matter, I just like dark matter)

This gives you a constant source of 3282 exp. (it sounds low but believe me, it adds up really quickly and really nicely) each time you go through the room. For me it got everyone from like level 70 to ** level really fast.

One reason I really like this method is because each time you beat the cybot battle, you get a turbo ether, so whenever crono and magus get low on magic you can just give one to each ot them and voila! Back to full MP.
Another cool thing is that if you only use crono and magus, that leaves your third party memeber to be anyone else you want to level up.
The only downside is that it's four rooms away from a save point, but that's not a big deal.

There are lots of places to lvl up. If you really wanna hit 99 the most fun way, though, you'll play the game over and over again. With new game + and hidden endings, I've always done it that way.

Thing is, you only need to play through twice to get all the endings.

i dont know the best place to level grind, but in the mid-late stage of the game mountain of woe will do it, 100 TP for killing a rubble with 500 hp

but i'd say wither geno dome or the reptite village in 65,000,000 BC(dungeon in ds version)

The important thing about grinding is accessibility. Good battles deep within dungeons just aren't readily accessible enough to make grinding there worth it. The best I found (I've only played the SNES game, so maybe there are better places in the Dimensional Vortex) is at the top of the second room of the Black Omen. There are two Synchrites (I don't know the DS name, but they're the robots that kind of look like oversized R2D2's...). Luminaire, Flare, Darkmatter, or even Shock (at high enough levels) should make this a one-hit kill. Make sure everyone has on status-protective equipment, though, cuz the enemies cause chaos. There's a save point right next to this battle, and you simply continue on to the next room and then come right back to fight them again. They give 2000 exp, which is incredibly good for such an easy battle. Hope that helped. This is how I ground up to 99.

The two snowbeast battle at the end of Frozen Cliffs (Dimensional Vortex 65000000 BC) seems to be the fastest, but if you want to max out magic for all your characters too, running the start of Dimensional Vortex 1000 AD and charming the magic capsules should bring you pretty close to ** by the time you have everyone at max magic.

I don't usually answer the same question twice, so please forgive me. SpectreXIII doesn't seem to understand what I meant, however, which means that others may not have as well.

I meant that if you love to play the game; it can be more fun to go through the story over and over, than to run in circles for a few hours. I was not saying that this is the fastest way to hit 99. Only that if you hate lvl grinding, it's the least boring.

SpectreXIII was , of course, correct. It takes only 2 playthroughs to get everything. And if you are the type of gamer who likes to get 100% as fast as possible my strategy is DEFINATLY not for you.

My strategy DOES have one advantage over the others, though. This is that, you'll get the BEST gear for all characters without playing the 'monster battle mini game' for days on end. Or for that matter, removing equipment from one character to give it to another every time you switch out.