This is my own Top 10:
1. Gildarts - Self explanatory. Physically, he's retardedly OP. Crash Magic is disgusting and disassembly was able to SPLIT A BLACK HOLE. IMO he's stronger than Makarov unless we take into account Fairy Law which can't actually be used because he's not an actual enemy.
2. Natsu (LFD and or DF) - In his "plot fights" with Erza, she always beats him, but he's above her. LFD puts him roughly on par or like a percentile below Laxus, whereas DF puts him in Gildarts/Makarov territory.
3. Laxus - DF Laxus is terribly OP as well, definitely enough to overpower Erza. We already know Laxus is OP so there isn't much to discuss here.
4. Erza - Variety of powerful armors useful for almost any situation. She literally has a "Plot Armor", which is ridiculous. She's not above two DF users though.
5. Gajeel - If we include Iron Shadow Dragon Mode (Assuming he is able to keep it from GMG) then he shoots up a tier in to Natsu/Laxus territory (Pre-DF). Still powerful in his own right.
6. Mirajane - She's ridiculously underrated and way too low on your thread. Her Demon form is overpowering and her Sitri form can challenge the top 5.
7. Gray - Self explanatory. He takes on the lower tier people in the guild with quite ease, but gets stomped by everyone above him.
8. Lucy - She has more potential than she's showing. Loki+Aquarius and the spell she got in the Nirvana arc are powerful and I'm assuming she'll be even more OP when she gets the last two keys.
9. Juvia - She hasn't really got much development power-wise, sadly. Her water body is like pseudo-intangibility, and without intel she's deadly.
10. Elfman/Wendy/Lisanna - Versatile but underwhelming power-wise.