I would say Tien is at least SS1 level and that's being conservative. Cell defeated android 17 and piccolo at the same time and was the equal to 16, all of whom would have had the ability to one shot a SS1 at that point. When cell absorbed 17, he reached a level of power that a full force punch to the face from android 16 didn't even phase him. Considering all of this, the fact that Tien was able to even slow him down for a few minutes would imply that he was a lot stronger then a ss1 at this point, probably somewhere between ssj1 and android level. (this was also before the time skip between the cell and buu arcs where i assume he powered up quite a bit)
@rein: I wouldn't say that he was SSJ1 level, the only reason he was allowed to slow Cell down was for plot reasons, Cell would have walked right through it otherwise and destroyed the hell out of Tien, just like he did 16 and Piccolo
@JThree47605: If we pass it off as PIS he doesn't have any other feats that would put him at SSJ level, but if we take it as a real feat i still feel as though the fact that cell didn't either just deflect the blast or fly right through it implies he's still pretty strong.
@rein: The fact that he has no other feat to that would put him at SSJ1 level is why I wouldn't rank him up that high. Cell could have easily plowed right through that blast and demolished Tien, but for plot reasons he didn't. I guess Toriyama just wanted to prolong Cell becoming perfect until the right moment. That's what I got out of it anyways.
Tien=100% Frieza in my book, the difference is... that Tien has a better ultimate, A Neo Tri-Beam from Buu saga Tien would probably kill Super Goku(the one who fought Perfect Cell), let alone Frieza, Frieza first form will get stomped by Tien, Frieza saga Tien easily beat Burter and Jeice, Cell Saga Tien is probably as strong as 50% Frieza, Buu saga Tien is equal or maybe even stronger than full powered Frieza. With that said i think that Tien can win against full powered Frieza 8/10, due to better techniques.
A real battle would be Tien vs Meta Frieza, Now that would awesome :), hard to pick a winner there.