First off, there are no official tier rankings or DBZ-style power level measurements in Touhou, so this can be a difficult question to answer. Moreover, weaker characters are capable of beating stronger ones - the Spell Card system was designed as an equalizer in this regard, plus characters may have certain weaknesses that can be exploited, or aren't taking the fight seriously, or are holding back - so "character X beat character Y!" doesn't necessarily mean that character X is stronger than character Y.
Having said all of that...if I were to organize a tier list, I would put her in the second-highest tier. She's certainly incredibly powerful - she knows powerful magic, she's incredibly fast, strong, and tough, and she can apparently manipulate fate (although the limits and the amount of control are unknown). However, I would place her behind Yukari, Suika, Kanako, and Suwako in terms of power.
I would consider top tier to be Yukari, Suika, Kanako, Suwako, and probably Sikieki; second tier to be Remilia, Kaguya, Mokou, Sanae, Reimu, Sakuya, and Ran. Yuyuko, Erin, Yuuka, Yuugi, Utsuho, Flandre, and Tenshi are matters of debate (well, they're all matters of debate, but in these cases we rarely, if ever, see their true power displayed or are clear on its upper limits, so it's much harder to say).
In short, Remilia is one of the most powerful in Gensokyo, but not the most powerful.